Do physical measures stop or slow down the spread of respiratory viruses?

If properly used ... there's likely a small margin of reduction in spreading airborne viruses etc. Fluid flow physics (and observation) tells us that putting an obstacle in the path of a moving stream disrupts it, taking a little energy out ...while focusing the rest (depending on shape).

But masks are [effectively] never "properly used"

I worked in Singapore when H1N1 was a big concern

Company issued us optional dust masks to wear to "slow the spread"

They issued one mask per person

Of those who chose to wear them at work (on my floor it was about 30%), every last one of them would leave the mask face side up at the end of the day on their desk next to their keyboard so they could put it back on when they came back to the office the next day