I think any account which hasn't earned any merit in the last 12 months should lost a possibility to wear a signature. Perhaps it would contribute to a more quality posting on the forum.
This would be a shrewd idea if you look at it from one angle, but i do not see theymos putting any signature restrictions(such as this)anytime soon.
The onus falls on the campaign/campaign managers to do so, if they feel it'll help clean the campaign off bad posters, and I think top campaign managers look into this, they check the general posting quality, if it's good, then the user must have earned one merit in 12 months, hence eligible; if its terrible, with no merits in the last 365 days, then it'll be a no, except in cases where the account is inactive, which they hardly even consider.
How can a campaign manager judge that a comment worth a Merit or isn't if it is a subjective assessment system?
Most campaign managers(top BTC campaign managers) have been in the system for a very long time, some are merit sources, spam busters, DT members, mods etc, their cv speaks for them, thus it'll be a stroll in the park for them, deciding if a post is worth a merit for whatever reason possible.