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).

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