A forum that I participate in is run by a friend of mine. There are 3 other friends that we know from real-life who are members of the forum. One particular friend persistently tests the rules, his actions just reach the brink of being a rule-violation before he backs off.

Needless to say, this activity is tiresome - speaking to him in real-life brings assurances that it won't be repeated, but this is not reflected in his actions on the forum.

What is a means to deal with real-life friend forum members who consistently test the rules? In a similar manner, how do deal with rule-testing family members?