Avoid using negative terms like "restrict". Simply - do not list it in amenities! Do not even place the photo. In that case, guests will not expect it (or more precisely, AirBNB wil not show your property for those who look for the place with the garage).
Only if someone asks for the garage, then tell them that it is for private use. And if the guest is arriving at the period when you don't plan to go there, so the garage is available - well, let them use. This will be a big plus!