Well the query is running successfully now. But it's not deleting any records.
I created a test account and was able to delete it by targeting it directly with an equality check
WHERE users.username = 'testdelete'
rather than delete everything not equal with inequality check we've been using
WHERE users.username <> 'testdelete'
I've also tried the != operator with no success.
If memory serves, I've read certain joins may not return the expected matches, so I've tried inner, outer, right and left. But no luck here either.