First thing I would do is contact the member on record and request them to change their password.
Might want to ask them if they shared there password with anyone and tell them that other ip in question was banned.
My 2 cents:
Just arbitrarily banning a IP may ban other legit members.
Since IP numbers can be shared.
I know with my isp the ip used changed every couple of months. But in the last year it has remained the same.
