One thing I think that might work is like @everittdmickey is doing by muting them, but with a twist, once a week or every couple weeks, or even just once a month, make a post title The following people are muted from my site for comment spam, that way your regular readers will know who to add to their mute list.
You save vote power, and earn a reward for letting us know who they are. I think if more active authors did this the comment spam crap might go away. Downvote obscurity is still visible, muted spammers and bots are not.