i suggest choosing a usenet goup, much as the xnews "forum" is a newsgroup (subject line contains [xnews])
maybe no-ads should use news://alt.privacy.spyware as no-ads' forum?
actually more no-ads messages (as seen in google groups search) show up in netscape or ms news servers, but alt.privacy.spyware is most common (and appropriate) usenet group.
next choice? seeing this forum reminded me of voyforum. i've very occasianally used a voyforum. it works very similary to this one. AFAIK, voyforum has no weird adware, popup thing, but i may have dodged that because i was already using proxomitron when i first visited that voyforum.
this list probably has similar old-school style bulletin boards: http://www.freebielist.com/messageboards.htm.
other than that, i'd avoid phpbb or those other boardwares that use obnoxious sid, s, sessionid, ugh. (my users perspective :-) )