[Mordekay]

It is way more simple to simply ignore those who don't want to give you the demo in question.
I'm running servers since around 15 years, this is one of the first lessons I've learned.

Yes it would be possible to limit the amount of demos recorded for each person, but not that simple. You'd need to create a database or table where every user gets added to stored by STEAM_ID at best and add one each time he connects. But... what if the limit is reached, the player in question connects, no demo is recorded anymore and then he does something against your rules? You'd be without a demo of him. So it is way better to simply ONLY accept the last demo recorded, which is the one before the player got banned.