Your position in the SERPS isn't what gets your keywords kicked out from the low click index. What is probably getting your words kicked out is the fact that for X amount of impressions your ad is only getting a low amount of click throughs. So, if your ads don't get click thru's then Overture isn't making money off your ad, and they delete it.
This being said I really doubt competitors would have a bot for this. Having a bot click on your ad would only increase your CTR, making your ads stay, and if they are artifically increasing impressions with a bot that means their keyword listings would be negatively affected as well (unless their bot was click on their own links only, which would cost them money).
I'd just suggest targetting your keywords better and write more effective/enticing ad copy.