Small suggestion

Small suggestion — add a limit to the delete query or make it an option. In case a site has built up a lot of spam before installing your plugin, running a delete all could crush the db server.

Limiting it to a few hundred each time it runs should help with that. It may take a while for your auto-deleter to catch up, but it should get there eventually.

http://ww.wp.xz.cn/plugins/bbpress-auto-delete-spam-replies/