SpamSieve is working great for me, capturing gray, purple, red, and yellow messages into my Spam folder “On My Mac” (maybe other colors as well but that’s what I’m seeing now).
Right now I need to manually select all those messages and hit ‘delete’ to banish them forever. I would like to be able to do this automatically with just the gray messages which I assume are the spammiest. I tried adjusting the “SpamSieve [blue][gray]” rule in Apple Mail from ‘Delete Message’ (which is how it is now) to ‘Move Message’ into one of my trash folders but nothing has changed.
Mail rules do not offer a way to completely delete messages. However, you can set it to move the most spammy messages to the trash, and you can set Mail to automatically empty the trash.
You might want to experiment with the Mail Act-On plugin (extension? or whatever we’re supposed to call these things now). It massively extends Mail’s rules functionality.
I have a Mail Act-On rule which I trigger every few weeks (whenever I glance over the 2000+ messages that have accumulated in my Spam folder) that “deletes” messages instantly, bypassing Mail’s Trash folder (there are a lot of normal messages which I like to let rest in the Trash folder till I’m sure I won’t need them again). I have this rule configured so that it only works on messages that have been marked as spam, as there is no “undo”.