If you go into a cafe and they charge you £1000 for a cup of coffee, you don't have to pay unless the price was agreed in advance. If they charge you £4 it may still be an expensive cup of coffee, but you have to pay. Likewise, if you impound an aircraft because the owner failed to pay a £75 landing fee, that may be OK, but if you do so because they failed to pay a £3000 landing fee I think you'd be on dodgy ground.
Off topic but in any eatery of any kind at all you are only required to pay what you reasonably consider to be the cost of the ingredients on the plate/in the cup. You could in fact pay about 15p for the coffee and be quite legally within your rights to leave without any recourse available to the shop.
If you want to charge someone money for something you can but you need to advertise a clearly displayed scheme of charges.
As far as someone landing on your property as much as you can do is report them to the CAA and perhaps prevent them from departing again by obstructing the aircraft, should you so want to, if it was me I'd tell them what would happen if they did it again. I'm fairly sure you can't charge them money without displaying some kind of scheme of charges but that would also require a change of use of the land and that makes it more complicated.
Basically, if you don't want to use the strip plough it, if you do don't and I wouldn't worry about people just using it randomly, it's a very unusual thing to happen
Chill out guys.