Google can ban your account at any time for any reason. Actually, they don't even need a reason. Read the TOS. But scaring people into thinking they will get banned JUST BECAUSE THEY HAVE A HIGH CTR is a bunch of baloney. As I said before, if your clicks are valid and you're not violating any TOS rules, you will not get banned for a high CTR. I've never heard of anyone getting banned for a high CTR . . . IF THEIR CLICKS WERE LEGIT. If you get well-targeted ads and your visitors are hungry for more after they've seen what you have to offer, they will click on your ads. In such a situation, high CTR's are indeed possible. But, if you're afraid your account will be banned for a high CTR, then you should take down your ads when your CTR exceeds the 0.3% to 3% range you seem to feel is OK. I'm betting you that NO ONE would do that.
Yeah- I have a couple of very small sites that are always around 30-40%, and a humor site that is more like 7% (by the way, it was 2% till I redid the look of the google ads!) There's nothing automatically suspicious about high click thru. But anyone doing click fraud- big Grother will find out sooner or later, and they'll lose more money than they would have made honestly...
It depends on the site, sometimes viewers just catch on to things easily. I'll go with 5% or less typically for a website that doesn't spend money on advertising.
Like Jack said, there are too many variables involved. But from my personal experience, anything from 3%-5% is what I consider good. Anything else is just extra.
30% does seem a little high. I would watch as it is above average and Google may take notice of that.