I know the leading crop-circle makers here in the UK, and it’s possible I may have been involved in crop-circle making myself (but there’s no proof). The situation as best I understand it…
When crop circles were new, yes, some farmers were able to cash in by charging for admission / viewing and may have made more money than they would have realised from the crop itself. This may still happen with some major formations, but it’s getting rarer as the years go by. The novelty has worn off for most people, and farmers are rarely inclined to disregard the ‘vandalism’ because they can make money out of crop circle tourists.
The National Farmers Union (NFU) has publicly condemned the activites of crop circle makers and has offered substantial rewards for the apprehension and successful conviction of a circle maker. With very rare exceptions, they don’t like it, don’t think it’s funny, and just want it to STOP. I’m not aware that NFU members have ever successfully caught a circle maker, although I do think there has been at least one case of a circle maker being taken to court and fined.
Some circle makers have, in the past, paid the farmer for permission to do their stuff, but this is usually only when a TV or film company, or an ad agency, is coughing up the money. In the vast majority of cases, making a crop circle certainly involves breaking the law of trespass, and a good lawyer could probably make a case for criminal damage or vandalism.
A farmer would be entitled, under the law, to try and get circle makers to cease and desist their activity, and to enforce the laws of property and trespass, but would not be allowed to use more than ‘reasonable force’ to do so. This is intentionally not defined in law - it’s for magistrates to decide what it means in each case and whether someone has used ‘reasonable force’ or not.
Gun laws are pretty draconian here in the UK, but I believe farmers are allowed to keep shotguns with a licence, solely for dealing with vermin and wild animals. It would be utterly illegal to even AIM one of these guns at a circle maker, let alone actually use it even in warning, and in any such case it would be the farmer who would go away for some serious prison time.