Create decoy profiles to monitor large-scale spam activity. Ban members who send spam to these profiles.
Monitor more closely accounts from members that have no information on their profile.
When a new member signs up, in addition to the traditional questions (experience, fields of expertize), add one question such as "what is the result of (49-15)x(59+945)/(9+8)". Monitor more closely members who provide the wrong answer.
Monitor more closely brand new members with blank profiles, or brand new members with irrelevant / spammy links on their profile (better: delete these accounts).
Members should not accept friend requests from other unknown members with a blank profile. Also they are encouraged to report spam issues: spammers, once detected, are terminated and this in turn clears the spam message on everyone's profile.