How to deactivate a hopper without activating a dropper directly below?

It's impossible not to power the dropper, but it's possible not to make it fire. All you need is not to provide a block update, which would make the dropper check for power and activate.

This example setup works - a solid block does not produce updates even when powered, locking a hopper does not produce an update, and a repeater will produce an update 2 blocks ahead, but it updates the hopper, not the dropper.

Of course at this point the dropper is BUD'ed, which means any update, from any source will make it fire. But that's the most you can do in this configuration.

This is impossible, because of the bug MC-108 (currently closed as "working as intended", but the proper resolution would be "won't fix"). A dropper always activates when the position above it is powered and it gets a block updates (which it always gets from the state of the hopper changing).

You need to work around it, for example by keeping the dropper powered from the last use or by putting the hopper on its side.

Power the hopper indirectly with a block.

Edit:

I did some more testing and using a redstone block has proven to be much better because the dropper won't activate if a block update occurs. However, the block can only be above the hopper.