I offer the simple decision against "Failed" garbage (spam) in transactions - let the game penalize each failed transaction by a small amount, sample 1 SD or (0.001 SD ^ n) for every fault, n is fault's number in the continuous sequence.
Human players will receive a small damage from several penalties and in time will stop, but owners of bots should use cleverer bots (if want not to lose stardust savings).
#Greenpeace is live!
Your solution is pretty smart. Very interesting idea.
First we have to make the gameplay much more comfortable - especially with e.g. 100 planets. We have to do our homework. Bots are already fading away since we tax a couple things. We can't make a emergency braking yet, there are still too many deficits on our side.