You are allowed to rent your signature space for ads. That's why there are so many signature campaign out there. I've been renting my signature for over a year and never been banned.
You are banned because you were making insubstantial posts (spamming the forum) to earn BTC from signature campaign. That's all.
I checked his recent posts and found him making quite a few posts daily:
https://bitcointalk.org/index.php?action=profile;u=263073;sa=showPostsIt appears he made less than 10 posts a day and many posts are more than 2 lines.