Keep it closed. Introduce genes only by AI, embryo transfer, or hysterectomy and fostering.
Purchase only EP free stock from a reputable seed stock supplier, if possible from the same source herd every time, or at least from the same breeding pyramid. (It would be sensible to check whether the source herd is also free from lesions of App).
Isolate incoming pigs for eight weeks and check that the EP free status of the donor herd has been maintained before moving them into your herd. (This is good practice whether your herd is EP free or not).
To be ultra careful, mix sentinel pigs from your herd in with the new pigs in isolation, one to two weeks after their delivery. The sentinels can be slaughtered and their lungs examined before the pigs move in or they can be blood tested on entry to the isolation and again five weeks later. (Whatever the health status of your herd, EP free or not, it is a good practice to put pigs from your herd alongside the isolated gilts. This helps them to gradually adapt to the health status of your herd).