If you shoot down every craft involved in the terror mission, you can delay the mission for months. Eventually the last ship, the terror ship, will come in, but if you already have global radar coverage, firestorms, and plasma beams, you might inadvertently be stopping them as well. This is because when you shoot down any ship, its mission is put on hold for a random delay time within a range set for the mission. No ships will be spawned for that mission until the delay timer is up.
You can test for terror sites by running a game and not shooting anything down. You should get a terror site every month, like clockwork.
If you wanted to avoid terror sites as much as possible, the best thing to do is to shoot down every small ship you come across, and even shoot down any terror ship you can catch. You'll want to get hyper-wave decoders ASAP so you can see their mission (also better detection), but terror ships often slow down to lower than interceptor speed before making the site, and can be shot down safely with 2-3 interceptors using avalanche missiles.
