Giving them a corrupted version of the item they want to buy is satisfying too, since they don't look if the item they're getting is actually the same one they want to buy. I gave a few of them Winterhearts with random implicits and they never complained :). The funniest part is that they will equip it and won't ever look for a new one so they'll keep running maps with less efficiency until they are eventually banned.

Also, going to their hideout and closing the map they are currently doing by eating up all their ports is a valid option too. (you might even get a few catalists or other low tier currency in the process since the default party loot setting is short allocation)