How about we give it some time, 90-120 days to let the temp bans expire and see how the sig-less copy-pasters perform. Will some of them disappear due to not getting paid? In which case the sig ban is good enough, as it gives good users a second chance and effectively bans the financially-motivated ones. Will they abuse the sig ban? Not sure how they would do that though.
If someone is financially motivated, he would not post as like he did before the ban. It would be easy to track if the list of users are limited. But from what I observed, there are hundreds of banned people and it would not be that easy to track their activities.
I would say sig ban must be of at least 1 year and if in case any user continue to contribute to the forum despite their sig ban, some reputed members vouch for them as well after some months, their ban can be reversed.