Just thought of bringing up to you notice that this forum misses two security things that may be a vulnerability to spams.

First of all, you have disabled E-mail verification option. So practically anyone can enter any data for email column during signup. This is the main vulnerability since the spam bots would target site missing this function and create a bunch of signups with some link to sites in profile.

The next vulnerability is the poor captcha image verification pictures. The kind of image you use in this site is much easier to be identified by automated spam bots. Try to get it replaced.