It would make sense to remove spammers' Discourse accounts rather than leaving them behind. I usually do this by hand at the moment, but bitmap suggested having a cron job that would remove them automatically if they have been marked as spammers for a while (removing them immediately seems too dangerous in case of a misclick).

There would be many ways of doing this, but the simplest might be to have the cron job run every week or so, and list spammer accounts into a temporary table the first time it runs; any accounts in that table still marked as spammers the next time it runs get their Discourse account deleted.