Spammy subscribers are no good! Here are a few suggestions for dealing with them:
If you've got an older Buttondown account, you may not have enabled double opt-in. Turn that on in the Danger Zone section of your newsletter's "Settings" page. It'll help cut down on spammy subscribers.
We'll be building out Captcha support in the future but in the meantime, if you still are seeing lots of spammy subscribers contact us and we'll see if we can help.