Qitzur Shulchan Arukh – 179:10-11
Someone who has a debt contract [i.e. an IOU] on his friend, and the contract is fading and heading toward erasure, he should go to court and they will make him a permanent one.
It is prohibited to leave a paid off [IOU] contract in one’s house, as it says “[If sin dwells in your hand, put it away] and do not give home to unrighteousness in your home.” (Job 11:14)
Note that the sin in se’if 11 isn’t that of theft, of using the contract. Just having the temptation around, or the possible chance of error (e.g. if the former lender dies and his heirs try to collect) is itself prohibited.