You can grant, retract, move add and change whatever you want.
As some others have already said, this isn't exactly true. Lets look at a scenario. You gave the guys permission, the one guy does not have a legal gun, so he goes and buys the cheapest one he can. Turns out he runs across a legal 30 carbine cheap and buys it. It doesn't sound like any consideration was exchanged, so you don't have a contract, BUT you do have a promise a reasonable person could use to execute the decision to purchase the 30 carbine for the specific purpose of hunting on your property(or any other decision). Enter promissory Estoppel/detrimental reliance.
http://en.wikipedia.org/wiki/Estoppel#Promissory_estoppel
I'd guess(I'm not a lawyer and certainly not a judge with jurisdiction OR both sides of the story), legally you aren't on strong legal ground if removing someone for using a 30 carbine when you did not stipulate that to begin with.
Making all that irrelevant is first the fact that one guy was using a rifle that was illegal, assumably with knowledge of the other. So in reality they were both breaking the law(even if they didn't know it). Second, with the amounts we are talking here it is never going to court, probably not even small claims, so what does the legality of it have to do with it?
Morally, IF the guy was breaking the law, and you don't seem all that sure, but no one from Minnesota has contradicted that fact, so he probably was, you have every right to toss him as he is unfairly putting you in legal jeopardy. If you continue to allow them to hunt with an illegal rifle on your property YOU are breaking the law. You have the right to call the game warden if you want.
Now I would consider the fact that these are "young" guys. They probably don't have a safe full of guns. They probably either have one or two OR none and they borrowed what they were using from family who were using the proper deer guns themselves. Likely those family members told them the rifles would be fine. They may very well have no firsthand knowledge of the regulations.
If they approached you at the grocery store, I assume they are an acquaintance of some sort. If so I think I would actively attempt to contact them and reinvite them with the proper equipment and possibly some other tips. Hunting property is near impossible to come by in my area if you don't own it yourself. My first shotgun, an H&R single shot 12 ga, is still in my possession as I lend it to new hunters(I put a kick eez pad on it). ODNR releases a rulebook each year available at every hunting store for free. If a similar book is available in Minnesota provide them with a copy in a friendly non-judgemental manner along with any stipultions you have for land use.
In Ohio ODNR has a release you can use, but Ohio law supposedly codifies landowner immunity from liability automatically(at least if there is no payment).