Okay, but as Hans points out:
To build and deploy at TL1 an improvised explosive device capable of inconveniencing a starship is not a simple task. ...
(emphasis added)
Let's understand the hurdles. It has to be able to do damage to a hull that is supposedly strong enough to withstand WW-II tank guns; that alone is quite ambitious for any explosive agent within reach of a TL1 society.
And, it has to be able to project that power far enough to control the area you're trying to protect. All the ship needs to do is land in the middle of the lake out of range of your measures to make your effort difficult to impossible - or hover over the lake and drop a hose down.
AND, it has to be available in quantities sufficient to cover diverse possible locations where the trespasser might set down. There's a huge gap between making a destructive agent and being able to accurately deliver it in the face of defensive efforts by the target, which - again, as Hans points out - can be as simple as moving to a different part of the lake or moving to a different lake.
I can build a dandy trebuchet at TL1, load it up with whatever hostile agent you guys decide on, maybe even deliver it with good accuracy.
https://www.youtube.com/watch?v=RN18I-RBVm4
What I cannot do is give it the range to protect more than a very small body of water. If you want to say what goes on more than a few hundred yards from shore, you'll have to row a boat out and hope he isn't gone before you get there - and if he lands in the middle of something the size of Lake Huron, you're just never going to know he's been there unless some boat just happened to be in the vicinity when he does it. There simply IS no power to extend jurisdiction beyond about a few hundred yards of shore at TL1 short of boats, and those will be limited not just by their visual horizon but by the power they can carry and the speed with which they can deliver it. Heck, just patrolling the entire shore against such an intrusion would be a daunting undertaking.
And unless that's the only lake on the entire blessed planet, all he has to do is move a ways off and he'll be fueling somewhere where they've not yet received word of his previous encounter. Lands, begins refueling, "Wait, you can't...", "Oh, I'm sorry, ...", takes off, moves a few miles, lands, begins refueling, "Wait, you can't...", and so forth, repeat until fueled.
At the end of it all the man could come by, suck up the water, go laser the local port town, and then come back next month to a different part of the world where they never heard the tale. Such is the nature of TL1 communications.