Hi,
Most mail, or server admins have a way or two of finding spammers, and in general, multiple emails from the same place (for example) are already looked at and addressed. Trolls is a bit different, and it requires an admin, to the particular forum that should be able to track usage ... for example, here Bapu is a good guy and help, but in some places he might be considered spam/troll and taken down. However, the main difference is that Bapu is an "active" user, and very helpful very often.
The really good spammers/trolls, that work full time at it, will spoof the headers some 10 to 20 times, and get away from it, for a bit longer, but eventually they get cut down.
It's difficult to catch everything, though, since you end up blocking way too much, and that slows down the whole process, way too much, and you and I would not like it. You kinda have to find a reasonable balance so you are not taking up processing time, and hurting the whole thing. I've always thought that a bot should be created to send multiple emails from the same person/site to a different check in point, for the purpose of checking ... and slow THEM down, not the rest of them.