I sometimes use imagery of a toilet flushing and the thought going down, several flushes if necessary (imaginary water is free!) Also, imagining white noise (visual and auditory) washing out the thought, increasing in power until it wipes away the thought like waves on a beach erasing patterns traced in the sand and then fading out, seems to work well. (I suspect using (imaginary) noise might have some actual biophysical basis for why it works, but that's just speculation.)