I'm trying to allow my users to change their user title (under their display name) to whatever they want if they have more than 5000 posts. Any idea how this would be done?

Not sure what version you need, but here's the 6.4 mod:
https://www.ubbdev.com/forum/showflat.php/Number/103583

If I did that the spam would never end

I allow it with over 7000 members and rarely have a problem