First, bonds As such are not illegal.
There is enough details in posts here about it, you can read and understand under what circumstances it is allowed.
You have not given any details of the bond or your employment period, so I assume it's a legally enforceable bond.
No one can force you to work where you do not want to do so. No one can force you to work after the notice period irrespective of whether they have accepted the resignation. The bond only defines the amount you need to pay in order to compensate the company for training costs.
So what you need to do is to send a resignation in a manner that there is proof of it being sent. Send again by registered post if necessary. Then serve your notice period. Intimate them about your last working day and ask for relieving process and handover.
After the last day, or on the last day, send a letter (again proof of delivery is important) offering to pay the bond amount and ask them to clarify the exit procedure, to,whom the cheque is to be sent and ask for your experience certificate. Don't send the cheque till they officially reply to you and confirm that the encashment of the cheque will be and official release.
Only remember that if your future employers want to do a background check, you will have a problem, so be upfront of what happened here.