Well... it IS your property... so.. research this first before you do it.. but.. I have to imagine there'd be no law against you doing something such as rigging up an exploding indelible ink bomb like they do in bags of money, on the bank etc. In order to prove this particular human did indeed trespass and was where they shouldn't be.
You could also make trap-pits. I obviously wouldn't put spikes in them... but at least make them big enough and deep enough where you know without a doubt if MrFr fell into them they'd get EFF'd up so they learn their lesson in a big way.
If you catch em in the act... maybe a paintball gun, where you've taken the liberty to first FREEZE the paintball ammo so it really freakin hurts upon impact?
Why wouldn't you confront them? Who cares about the time of day? Just like hunting, with proper preparation it should be relatively risk free. You'd have the tactical advantage of knowing of their presence before you engage with them. So you'd totally have the upper hand. If it's a concern you could reach-out to others before you engage for backup just in case.
The game wardens would do something, that's potentially money on the table for them in terms of fines. No way their bosses are gonna let free-money just sit on the table without wanting to grab for it.