Throttling a pump (with ball valve) really depends on the type of pump you have.
Centrifugal pumps (almost all pond pumps and powerheads are centrifugal) are fine to have the outlet throttled, just make sure you don't restrict the intake! Running them dry should be avoided at all costs, damages the internals (impeller, seals etc...) and is noisy as ****!
Completely shutting the valve on the outlet side shouldn't cause any harm to the pump. If they've been designed by anyone with half an ounce of knowledge on pump design they'll easily be able to withstand the shut-in head caused by blocking the outlet, that wouldn't concern me.
As far as throttling the outlet being good for the pump? Well perhaps... I'd like to see some evidence before I believe that, but certainly it's not bad for them
However a positive displacement pump is a different story entirely, but these are seldom seen in pond supply shops!