[Ryan Hellyer]

If you don’t integrate the users database then bbPress will only be installed in it’s own section of the database, or it could have an entirely separate database so if you do it that way then it’s just a matter of deleting the bbPress specific bits.

If you integrate the user tables then the two sets of users will be merged and removing the bbPress added users could be quite time consuming if you have many forum users. I assume (not 100% sure here) that the only way to remove them would be to manually remove them or to rebuild the WP ones from scratch.