How about instead of taking out repair points, take out "Red Line" Points. Make it where if your not engaged in combat ( outside the red line ) You don't earn Jack Kittens ( btw I did say kittens not the word you were thinking )

Unfortunately that wouldn't guarantee an end to farming.

A farming squad could hike out to the edge of a big skirmish map and set up camp, leaving you to compete with half a team. The opposing force would have little incentive to leave the capture letters to seek them out as long as they were smart enough not to spam PS's. That's assuming the OpFor even knew what the farmers were doing which is unlikely in the heat of battle.