Well, one way to discourage players from renown farming could be to cap it. Instead of being a continuously rising value that grants benefits every few points, it could grant certain benefits at a fixed value. In order to not make the system obsolete after reaching the cap the value could decay from certain actions or inactions (read decay over time).
Alternatively, instead of capping renown, how about a diminishing returns forumla? You gain prestige fast at the beginning when you have none, but the more famous you get the harder it is to becoe even more famous. Someone with 800 renown is already a superstar, defeating massive armies with just a handfull of troops himself is pretty much expected of him, he's done it countless times before. He gains maybe 1 point of renown, if any, for a fight, whereas a newcomer with 100 renown would gain maybe 10 renown for the same fight.