Assume a bank gives a loan to Mr X. for, say 100. The bank's assets (account "loans") increases by 100, and its liabilities (account "deposits") also goes up by 100 (which is the extra 100 added to the current account of Mr X. 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 use the law to reclaim the money and interest (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? Wouldn't this leave the bank exactly the same as in the beginning?