How do I bulk delete regular members that have been marked as Spammers? I have hundreds and they show up as members (albeit flagged). 

Why exactly are you looking to remove them? It is a good idea to keep them so they can't re-register. 

Inflates user metrics, pollutes data that uses member as a criteria (membermap as example), can confuse legit members who aren't really aware of the spam flagged status?

A couple hundred are from 2012 and doubt those are lurking, waiting for an opening to re register  

Bulk moving them to a different group could help. 

You can use the member search tools to mass prune or move members, problem is there is no filter to just get "Flagged as Spam" users.