Sourceforge uses phpBB, which is the most spam-prone boards I've used.

More moderators will help for sure, but this can't be unique to LMMS.  See how other larger community projects are handling the sourceforge spam and see what their solutions are.

It's hard to argue for an independently hosted message board when sourceforge provides one for free, but the sourceforge forums don't give the control needed in terms of human verification and forum updates that's needed for effective spam control.

I once eliminated 100% of spam by asking new members a human question (such as "what color is the sky?")