How do I prevent a tube valve from fusing to my rim
Sounds like galvanic corrosion. If you're somewhere that salts their winter roads, this is why people ride beater bikes.

A fix might be to smear the valve stem and around the valve hole with a thin layer of RTV silicon or possibly a grease to stop the two surfaces coming in direct contact.

Combining brass and aluminium is generally considered incompatible 

You might prefer to choose tubes that have rubber up the sides of the valve stem (not uncommon on Schrader but rare on presta) or some tubes use stainless steel valve stems.

To be honest, I'm surprised there aren't more corrosion problems between alloy rims and brass valves.