After playing a while I've found that interactions with neutrals are pretty lacking and sometimes very weird. While cut diplomacy is fine (we interacting with non-empire anyway), it leads to some unexpected consequences. Especially absence of declaring war option. Why is it bad? Well, there are numerous cases when I was able to refuel at the neutral colony or get load the resources while invading them with my troop transports.
What do I suggest?
Add Declare War option to diplomacy interactions with neutral colonies. The surprise invading leads to instant declaring war from an invaded neutral colony. While at war you can't refuel at the colony or trade, all past trade treaties are cancelled. Peace is possible after gifts.