I can't believe I've missed this. (Maybe because ships are less likely to be going one.)

Is there anything that would prevent an opponent from using Comm Noise to reduce your speed from one to zero (and therefore stop you from using defense tokens)?

Nope.

There is no "To a minimum of One" ruling on this card, as a counterpoint to Tractor Beams...

I did it 3 weeks ago to a Victory Star Destroyer on Turn 2... It had moved up just into range, and I rolled Double Hit, Double-Hit, Crit, Acc on my XI-7 wielding Assault Frigate who was speeding past at Speed 3...

He was not only out of arc at that point, but as he'd activated, it was turn 5 before he was moving again, and completely missed the battle...

I do this all the time!

I can't believe I've missed this. (Maybe because ships are less likely to be going one.)

Is there anything that would prevent an opponent from using Comm Noise to reduce your speed from one to zero (and therefore stop you from using defense tokens)?

I had this done to my ISD flagship - it was very painful :'(

Yeah...I took down a kitted VSD2 flagship this way in a wave 1 tournament...wasn't pretty.