Rather than banning, they should simply be moderated/regulated. It should be 1 post/day max since that's = 1 activity. Then, if sig camp posts get merit, there should be bonus. It seems useless to post more than once a day but there are days when someone probably is "in the mood" and has a lot of great ideas, just like writers, so.
Here is what will happen if you add rewards directly for merits. Suppose there is reward of $2 worth bitcoins for one merit received. Members will start buying merits for $1 each then and pay it from the bonus. It will also affect the merit system on the whole as those members may rank up after accumulating merits in this manner. Most of the signature campaigns have criteria that only eligible posts will be paid. Campaign managers go through the posts before paying and thus those posts are not a problem for this forum.