fathom - 6:45 pm on May 13, 2013 (gmt 0)
Re-tool the signature code so that the link on the member's profile page is the only external link and their massive internal forum contributions that built value for you over time (compelling content) is rewarded by providing their profile solid link juice.
THAT would make you look even more like a spammer. Most spammers are not interested in building local following at the forum site and only register to have that one profile link. If you were a member for years but all you have is also that one profile link, you will look nothing different to Google than a spammer that just registered a second before Googlebot hit.
Many profile page URLs are robot disallowed or noindexed just for the very purpose of discouraging drive-by registrations for link dropping only. I do it on all forums I support. I don't know exactly how Google treats signature ULRs on forums but it's pretty clear to me that if you are consolidating all of your forums links from that site into one profile URL, you might as well just simply delete all the links.
Much like profiles are not default set to disallow, you have to actually implement that specifically... just like you can implement specific features for trusted members by membership time, post count, REP level or whatever and/or a combination of them and the lack of features for others like drive-by registrations.
Clearly as a forum owner the onus is on you to set your priorities for the audience you wish to serve.
Be that as it may, the only link you get here is in your profile... go figure if that is as big of a problem as you elude to.