IMHO, if he/she is willingly paying for the right to be a member, I'd think regularly reminding all paid members of the value that is available to them as sufficient. In the case of a member that has not participated at all in 4 months, you may want to reach out ... personally, to show your concern, but I certainly would not cancel their subscription unless they requested it. There may be a very logical reason they haven't got around to getting involved, or may simply need a little push to get going.