I roll secretly. The party at this point knows they are being scried on repeatedly. They've used See Invsibility to spot the sensor a couple of times, but haven't taken any other actions yet. I'm not sure they are thinking hard enough about what to do.
-Be invisible
-Have people the party is talking to use disguises or stay out of visual range of any sensor (they are in a hexcrawl so there's plenty of space for this, not like in a big city).
-Deceive the scryers with false information, perhaps by laying traps ("I'll bury the treausure here" when "treasure" really means "big pile of Alchemist's fire on a hair trigger")
-Use Mordenkainen's Sanctum (they do this when resting)
-Amulets of Protection against Detection and Location are merely uncommon, and there is an artificer in the 15th-level party. I'd have to run the numbers, but he can crank out enough for the whole party very quickly.
-Use Divine Intervention to request a smite on the scryer (the 15th level cleric has never used DI)
"As you complete the scrying ritual, you feel your awareness slipping away, hurtling across vast distances, until your senses snap into focus. You're in a small room, lit by a series of lanterns mounted to the wall. You see your target, standing before you, disrobed save for a towel around his waist. He approaches a mirror, before which lay a small bowl of water, and peers into it closely... then begins picking at his teeth."So bear in mind that this works both ways. Are you going to spoil the players' fun later on by having their opponents always sensing that they're being scryed on and going all coy, rather than just happening to break into a bout of spontaneous exposition?