I had a discussion with the administrators a while back.

Digging around a bit, the main problem with this forum is that the authentication systems (Captchas, that sort of thing) are well out of date and have been worked around, so there's very little preventing automated bots joining up and running amuck.

There are many other larger 3D printer forums without the spam. Even the Thingiverse forums are relatively spam free. Seems like most 3D printing discussions are on Facebook though, rather than dedicated forums.