We gotta be real here, folks.
We are talking about PEOPLE... the cleverest, most determined and destructive critter there is.
If they want to, they will.
So, gotta change their "want to."
Suggest a plan to create fear and anxiety in their mind...so they will "want to" go somewhere else and not on your place.
A fence will not stop them...not from entering the USA or leaving the USSR..
Use a single strand of high tensile wire and T posts to mark the boundary of your property. This can be taught barb wire or smooth. Use plastic insulators and a high impedance electric charger back at your house/shed which puts about 8 to 9 thousand volts on the line for short periods. Yes, they can go under or over it if they want to but you have marked your space.
Assume they are literate...Use the wire to hang signs on at intervals where you can never approach the fence without seeing at least one.
Sign should properly worded for legal notification for the country indicating trespassing is illegal, the property within the fence is private according to ..give number of applicable law..looks intimidating, and that hidden traps, poison and hunting are used within the property to control undesirable animals. (In Texas this is required when trapping/poisoning coyotes. You may not have coyotes there, but you do have undesirable people/animals). This way, you have marked your property boundary and given fair notice so you can prosecute as desired.
Now, you can also walk out there when you see vehicles parked and wander around shooting into the trees...suggest a 12gauge shotgun... makes noise as pellets rip thru tree tops... this is called aversion training....talk to yourself gesticulate and fire rounds in random direction...giving the appearance that you are completely daft. After doing this a few times, word of mouth that this is now a dangerous place should reduce the number of people mushroom hunting there.
In the early days when Texas was part of Mexico, a person claimed property by tearing up grass, throwing it into the air and shouting "this is my land!" This ceremony was witnessed by local folks. The new owner did this at all the boundaries of his land. Something similar may be lurking in the collective unconscious of the trespassers... perhaps you can escort them to the edge and perform this ceremony
With as many as 50 people there at once...there are a HUGE number of folks who are accustomed to coming there and considering it their mushroom field, so will take some time for word to get around that it is now dangerous and they are unwelcome there. It is hard to argue rationally with people who are trespassing, just ends up with you madder and them convinced you are unreasonable and they see it as a challenge to return without your awareness. Suggest you just behave in a crazy manner without approaching the people, just shoot over their heads , just instill fear into them so they leave the property voluntarily fearing for their own safety.
Best wishes... let us know how it goes... tough problem dealing with people's long held customs and practices...really, really tough...