If you really want to force the issue, you just make the all the sub-forums private except an introduction forum. Once a person posts, they get granted access to the rest of the site. This would take active moderation, but it's been done on many forums very successfully.
Might as well just do it right if you're going to do it. I'd even vote to implement that to get it over with lol.
1986 Kawasaki Vulcan 750
NGK Iridium Plugs #7803/DPR7EIX-9
Duralast Gold ETX15 AGM Battery
Coastered & Shaved
Metzeler ME880 [110/90-19, 170/80-15]
Balance Dampers Replaced
Rebuilt Forks w/ Progressive Springs
VN750.com Grill Cover
Emgo 23-92411 Handlebars
MOSFET FH012AA R/R
1990 Kawasaki Vulcan 750
1998 Honda VFR800 FI
2014 Honda VFR800F
1989 Pontiac Firebird Formula 350