Without the dog, I'd probably do the same as pretty much every farmer I've met when walking in that situation has done to me - give the people taking a shortcut across my land a little wave or a nod and a smile or touch my fingers to my cap. That does assume no history of problems and that there isn't a particularly sensitive species in my wildlife area, as neither was mentioned in the OP.
With the dog, however, that would force me to be more active. As I'd want to ask them to put it back on the lead, I'd have to stop and talk to them, which places me in an awkward position of having properly acknowledged their presence. Having done that, if I didn't point out they shouldn't be there, I'd be tacitly approving their actions which could come back to haunt me.