Yes, it can be legitimate to block someone for off-wiki activities.
Our
tradition has been to try to mostly avoid it as being overly intrusive into the lives of editors, but it really depends on the context and complex judgment calls. That's what the ArbCom is for. It is a depressing and difficult job.
--Jimbo
I do not consider arbcom decisions that diverge much from existing policies as legitimate. The [[Wikipedia:No Personal Attack]] policy states that off-Wikipedia behavior is a side-issue. If you do not agree with that policy then please try to change it. Arbcom decision that diverge much form existing policies are unfair and erode the moral authority that the arbcom now has.
Andries