Yes they can, in certain circumstances. They can also direct you to land the AC. While they don't, as you have said "control the airspace" they could sight plenty of other acts they have authority to administer including those relating to Public Order, Protection from Harassment, Sexual Offences, Terrorism, breach of the peace, public nuisance etc. You are best to explain your intent to land (probably, as I chose to do, explain you will be walking away from them to bring the AC in at a safe distance) and that you will then have a conversation with them. In my case it was not police (council ranger) and to cut a long story short the outcome was that in his view I probably wasn't allowed to fly where I was however he accepted he couldn't explain why or sight a particular offence. He suggested I enquire with council as to the regulations. Which I didn't and haven't had an issue since. I was over water, did not fly over people or within 30m of any buildings, cars etc and was within VLOS (sort of). Didn't matter who was wrong it wasn't, in my view, worth the hassle of an argument.