I take all my dogs, making a great show of making them walk to heel (ie they could easily be let loose) and tuck my air rifle under my arm (as i'm just off, on my land, to shoot jackdaws) or the gripe (mucking out fork) and I go and ask them what they think they're doing?
Sometimes they announce that they're going up to the bridge to photograph steam trains.
That's when my best school-marm, scaring-big-lads-into-better-behaviour voice comes in handy! 'is that right' I ask, my eyebrows shooting up into my fringe, 'where's the footpath?'
Usually by this time they're beginning to suspect they're on shaky ground and they start nervously asking permission, which to be honest, I usually give, having told them I don't expect to find people wandering on my land without having asked, and given them earache about dogs, gates and walls.
The bolshy ones I plant myself in front of and tell them they are not going any further and to remove themselves.
It's all bluff, I can't shoot them, my dogs are softies and I'm on my own - but they don't know that and I wouldn't argue with me

Re the mushroom-picker, it's a rare person who would repeatedly come back knowing now he's going to get grief, and even if he is one of those blokes who can't be in the wrong

at the time, he'd have to be far fonder of conflict than most people are, to come back.