The way it works is that new users still see the box to add their signature in their member profile - but the length of signature allowed is 0 until a member has 30 posts. This is to prevent people from signing up just to get links on the board or links in the board's database.

Thanks for your answer.
I fully understand that, I run two article directories (one is dedicated to real estate) and I get many spam trials as well.

One question though, after a member reach to 30 posts. Will the signature be shown on the previous posts as well?

Yes

Thanks for the quick (and wished) answer