I have a small tankless water heater and the flow from my bathtub spout is too strong and the water will not get hot. How can I reduce the flow in just the bathtub spout?
Get a shut off for the shower head (or one that shuts off) and try jsotola's idea: just don't open the tub spout fully. A shower head is a lot easier to take off than a tub spout (anything but that - you'd have to, to put an aerator).
I suggest you to add an aereator (aka economizer) to your tub faucet: it will, about halve, the flow used at a reasonable price.
If your water heater is smaller than 24 kW you should get a bigger one. (24kW allows 10L/min flow @30K temperature's jump) that is pretty low flow and not-very-hot water, if you live in hot regions it may be good (cold water hotter than 15°)