My friend is constantly being bothered by possums at his place, and they're even coming into his garage and attic. Is there a good organic solution for keeping the critters out (in addition to plugging up any holes they might be taking advantage of)?

In my youth, the most effective treatment was 38 grains of lead applied intercranially with a Browning .22 rifle, but the Grapevine PD has informed me that this particular solution will bring other unwanted visitors, wearing guns and badges...