What to do with ineligible ITC?

Yes. I want to know ho to reverse that.

While filing GSTR-3B, show this under ITC reversal tab. You can show it under table 4(B)(1).

But, If you are having good business, I would advice you to take opinion of your tax consultant/CA before reversing the same.

Regards

CA Sandesh Mahipal 

If I reverse it in GSTR3B, will I have to pay it in cash? Or the credit ledger balance will be simply deducted by the same amount?

At the time of filing of GSTR-3B the said inalienable ITC you need to reverse.