How to fix a leak in an inaccessible bathtub fixture?
The hot and cold control valves are generally serviceable from above. I would guess (can't say for sure because I don't recognize the brand and model) that you could pry up the circular cover on top of the knob(s) to access a screw that would allow knob removal.

From there, you should be able to disassemble the valve assembly by taking apart the packing assembly, unscrewing the stem, and replacing the worn out washer(s) that are causing the leak (make sure you shut off the water supply first).

This is by no means a sure thing, some roman tub (that term may help you with finding parts) faucets have other means of securing the knobs (like a set screw on the side), and some have proprietary valve designs that do not work like traditional faucet valves. There are myriad brands and styles both foreign and domestic. If you can get the knobs off, you could at least take them to a reputable plumbing supply shop to try to identify your unit and get more information to help you repair it..