If your spammers come from different countries than your main audience, you could just block that country or ASN. I've been doing this using TPU's antispam addon and it works wonderfully. Once they hit the brick wall a couple of times, they give up and go elsewhere, looking for a softer target. Then you can allow registrations from that country or ASN again.
Spammers are such scum. If, as Alfa1 suggests, they are doing this so to manipulate you so that you turn off confirmation emails, that only makes things easier for them and may harm you in the long run. Plus, it'd be a shame to give them the win.