I think I have heard of PunBB but I am not sure if I have heard of FluxBB. I just checked the PunBB forum and I think it is clean.
FluxBB was a fork of PunBB but it's no longer being maintained unfortunately. Here's what the support forum used to look like before it was shut down.
https://web.archive.org/web/20220707162749/https://fluxbb.org/forums/index.php
