Thats the problem with this, you can do anything about it.
Except be careful, build shield, etc etc. But these measures do not address the root of the problem with planting. Hence, you can’t do anything about it. You also cannot concretely prove it was a plant from another alliance, so its legal as far as legality is concerned.
As you can see, this issue is something a lot of people are speaking about/against. Pretty much no one likes it, except for those who use it or when they use it. It still happens however, because, you guessed it, you can’t do anything about it.
Now for some counter-measures that can reduce this pain in the ass:
- Build a shield (pretty easy)
- Keep a watch on colonies near you. Like looking at achievs, nuking etc (moderate)
- Have at least 1 person watch the map, and the rest be contactable (very hard on small alliance worlds)
and some other, but I feel this post is already too long