Farmer-Bob:
Farming
Farming is illegal. The definitions of farming are, but are not limited to:
A. Attacking a target with his/her consent to get his/her asteroids/ships
B. Defending against someone who is attacking with their consent to steal their ships.
C. Abusing game features in order to gain excessive XP (e.g. roid farming)
D. Doing either a, b or c by using multi-planets
taken from:
http://game.planetarion.com/terms.pl
So yes, there's a rule against farming. It's in the Terms and Conditions we all approved when signing up to the game.