I think 2 BTC would be more appropriate if the forum was in some kind of need for additional money and it was for something other then to enable signature features.
There are plenty of ways that the money could be spent.
I might speculate that the volume of accounts traded might go up as people might be interested in buying accounts at fire-sale prices, hoping that signature campaigns would make a return appearance, giving value to accounts once again.
So, the right approach is not to completely ban them, but go in the lines of OgNasty's suggestion.
I also have observed many signature spammers decline to "invest" the small amount of time required to even briefly read a small number of posts in a thread prior to posting nonsense.
That is one of the main problems in their posting habits. They don't read posts, end up rewriting what someone else already wrote and they keep repeating this cycle. Their overall posting quality becomes trash.
Why is that foolish? If you charge an amount that is greater then someone can reasonably expect to earn via their signature over a medium amount of time then no one will pay the "fee" and you are essentially banning signature campaigns.
That is a hasty generalization. You can't know this, as an example I choose myself, assuming that staff members also lose this functionality, I would pay the 2 BTC fee.
If you pay 0.05 btc every month on the 1st, then you will be risking that 0.05 btc in the event that you post enough crap so that you get banned.
Risking 20$
Whoa, now I'm scared; I shall not spam anymore!
Exactly. There should be no ROI at all.
Why do you think this?
As long as people see posting as some kind of investment, and buying accounts in order to ROI, this will be a problem. The point of this forum was to initially discuss Bitcoin related stuff and provide help for people, not make 100 pages of posts in threads like 'Why is gambling bad?0 (note: 1 post was sufficient to answer this question).