You, on accepting this offer, undertake to stay in the employment of the company for a minimum period of 18 months.

Kindly confirm if this could be any aggrement on whose basis my company can forbid accepting my resign. I have resigned but company not accepting my resignation. Kindly help how should I proceed.

To me, such a condition seems to be vague as well as unilateral.
The offer already would contain, inter alia, the clause for unilateral termination of the contract by either side. As such accepting the offer in its entirety cannot create another condition pertaining to the same clause of termination outside it.

Sir, In this senario what steps should I take to get my resignation accepted.

The fact of the matter is that you knew well that soon after or while accepting the offer itself that you have to serve the company for a minimum period of 18 months. Therefore, before taking whatever recourse against the non-acceptance of your resignation by the company now, I would suggest in the interest of your future career that you have to find appropriate answers to the following pertinent questions:
(1) Despite understanding the condition couched in a plain language, why you joined?
(2) Perhaps if it were your dire necessity of a job then, how long you have been there now and whether got confirmed?
(3) If it is a substantial longer period like 9 months or one year, have you not developed good interpersonal relationship with your superiors in the organization?
(4) Whether you have got an offer of a better job on hand?
(5) If 'yes', whether you formally informed the present employer?
(6) If the reason is other than alternative employment elsewhere, why can't you wait till the expiry of 18 months service as you had already agreed?
(7) Whatever the reason for exit on your own, have you ever tried to approach the CEO and convince him the necessity for some relaxation?
(8) In case the company sticks to its stand, have you got the wherewithal, time and patience to fight it out legally?

Sir, if I abscond from this company and start working in anyother company could they do anything against me.
I have not signed any bond papers.
Also, there is no clause mentioned in my resignation part of the joining letter.

In fact, the battery of questions I raised was only to guide you to seek appropriate legal remedy and as such it is better that you choose to answer them in stead of asking further questions.

(1) Despite understanding the condition couched in a plain language, why you joined?
Sir, I asked the hr same point and she told that it is just a commitment that we ask from our employee and no issue will be there at the point of leaving but now she is jibing at her words and saying you have to server for 6 more months before we could release you.
(2) Perhaps if it were your dire necessity of a job then, how long you have been there now and whether got confirmed?
Sir, its my 4th month with the company and I have not received any email regarding the confirmation.
(3) If it is a substantial longer period like 9 months or one year, have you not developed good interpersonal relationship with your superiors in the organization?
Sir, its a small company driven by 1 man the director and no other person has any power.
(4) Whether you have got an offer of a better job on hand?
Yes, I do.
(5) If 'yes', whether you formally informed the present employer?
No, as they have told me that they don't want me to work in anyother company for after I leave this company and that is why giving the release after 6 months so that I couldn't join any where.
(6) If the reason is other than alternative employment elsewhere, why can't you wait till the expiry of 18 months service as you had already agreed?
Because once the 18 months complete they again will trap me in 1 year bond at the time of aprasial.
(7) Whatever the reason for exit on your own, have you ever tried to approach the CEO and convince him the necessity for some relaxation?
Done every bit.. even begged..
(8) In case the company sticks to its stand, have you got the wherewithal, time and patience to fight it out legally?
Depends on what stand they could take as I have not signed any bond papers and now I even don't want any releving letter from them.
I really apperciate your efforts for helping me.

In the absence any employment bond duly signed by the employee or any specific bilateral agreement to serve for 18 months or any alternative condition in case of failure on the part of the employee, in my personal opinion, such a clause becomes null and void ab initio and hence unenforceable in a Court of Law.
Of course, the employer, as a superior party to the contract of employment, may refuse to accept the resignation submitted any time during the 18 months period and if the employee leaves on his own without a formal acceptance of his resignation, may declare him to be an absconder and initiate disciplinary action against him on the charge of unauthorised absence resulting in consequential abandonment of job. Even if the employer prefers a legal remedy, no Court would grant any decree in his favor because of the absence of alternative remedy against breach in the alleged contract. And if the employee agitates against such a patent dismissal in a Court of Law, the Court would certainly set aside the orders of dismissal and the stigma on the employability gets automatically removed.
I think that good sense would eventually prevail upon the employer and he would realize that he can do nothing legally other than making some threats.
Therefore, the poster can simply walk out after submitting a detailed explanation for his decision.. As a pre emptive measure, he should appraise the prospective employer of this development and ensure his appointment.

Thank you so much sir for your help.