Is a neutral tag appropriate for accounts that are bought that have no reputation, or should accounts be ruined period?
There is no universal answer for everyone.
I think that neutral feedback is enough in most cases when suspected purchased account is identified, but sometimes negative feedback can also be applied.
Neutral feedback should be given if user purchased account only to participate in signature campaigns with low quality posts, with no real contribution to forum, and for making up fantasy stories and lies.
Negative feedback should be given only for high ranked members with prior reputation, when someone is caught scamming other members, or doing other suspicious activity in forum.