Could a bank simply eliminate a loan from its books? Assume a bank gives a loan to Mr X. for, say 100. The bank's assets (account "loans") increases by 100, and its liability (account "deposits") also go up by 100 (which is the extra 100 added to the current account of that who asked for the loan). 

Say Mr. X does not want to pay the loan+interest. The bank is, by law, entitled to them, so it can perfectly use the law to reclaim the money and interest back (maybe possessing some material wealth like a house or other collateral used to secure the loan in the first place). However, can a bank, in theory, simply erase the loan and deposit? Can it simply eliminate the 100 from both assets and liabilities? Would not this way the bank be exactly the same as in the beginning?

PS: sorry I am asking many questions. I am study economics and banking and cannot understand everything very well. It is great to have a place here to improve my knowledge
If Mr. X hasn't used the money, so it's all still in a deposit account at the same bank, then the bank may be able to cancel the loan. Whether they can or not, will depend on the exact contract attached to the loan.

If Mr. X has withdrawn the money as cash, then that transaction represented a drop in the bank's liability to Mr. X of 100, and a drop in the bank's assets of cash-at-hand of 100.

If Mr. X has used the money in a digital transaction, then they can no longer cancel both at the same time, as the asset of the loan is with Mr. X, but its matching liability will be with another party.

In either of the last two cases, the bank can cancel the loan, reducing its assets by 100. The balancing transaction will be a reduction in the bank's equity of 100 - i.e. the bank records a net loss on the loan of 100, and the shareholders make a loss of 100 on the transaction as a result.

(because assets = liabilities + equity)