I don't want to waste too much time on the same thing. TL;Dr; length limits are a very bad workaround that really don't have an effect and can be easily cheated. The length of a post is not directly related to its quality. A very short post can be much more useful than a very long one. This is not happening, let's move on.
allow only full or senior member and above to join signature campaign and allowing only fixed rate campaign will reduce scam a lot.
I think that it would be really effective if the limit was Sr. Member and above; there are already a lot of full members, albeit decreasing the spam for the newbies and jr. members would be good too.
Yes you can create 10 account but can you make them all senior ? NO
if someone makes new account it will take them time to get senior rank where they will allowed to join signature campaign
No, but it is fairly easy to 'level-up' 10 accounts to the rank of a full member. Additionally they could just go and buy them all and start spamming.