It's impossible to stop an airfield from being used unless you're firing thousands of missiles per week just to stop repairs that takes hours at most, realistically only a sizable nuke can cripple an airfield.
Depends on the infrastructure available, etc. if you don't have hardened shelters aircraft are easily destroyed on the ground, fuel infrastructure, etc. can be a significant problem. Ukraine doesn't have a clear run of it - a few (accurate) missiles a week will impact the serviceability of their airbases and ability to perform missions even if repaired in hours.
I don't think it is the biggest problem though, they still manage to maintain some air capability now with only sporadic, often inaccurate, strikes against them - though that might change if their air force became more of a threat. The bigger problem IMO is whether Russia can escalate the air war, which is a somewhat debatable one, but on paper if Russia was forced to escalate the air war the change to the overall dynamic of the war may not be in Ukraine's favour, at least as things stand, while they can't take the war into Russian territory.