[Himself (OP)]

This is probably a stupid question, but is there any way to prevent water from freezing in sub-freezing temperatures, i.e. putting it under so much pressure it cannot expand and as a result not freeze?

[ScientificSorcerer]

put it in a vacuum.

[RD]

... is there any way to prevent water from freezing in sub-freezing temperatures, i.e. putting it under so much pressure it cannot expand and as a result not freeze?

Yes very high pressure can lower the freezing point of water below 0^oC ,
but requires pressure in the region of 100 to 10,000 times atmospheric pressure  ...

 [ Invalid Attachment ]
https://en.wikipedia.org/wiki/Ice#Phases

put it in a vacuum.

Water freezes at a very slightly higher temperature in a vacuum than at atmospheric pressure.

Being in a vacuum (e.g. in space) does not stop water turning into ice ...
 

The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia

https://en.wikipedia.org/wiki/Comet#Nucleus

[evan_au]

If the water is extremely pure, and is held in a container with no nucleation sites, you can take the temperature of supercooled water as low as -48C/-55F before it freezes.

You can put salt in it, or add antifreeze.

Some arctic/antarctic fish produce natural antifreeze so they can continue swimming in "freezing" salty ocean water.