You have the option to refund a station within 24hrs of purchasing. You must however have the original vehicle type at the station for it to give you the option to refund 
There is currently no way of turning off specific missions, but you could always select the non-transport option when the mission asks you to transport - it awards half the credits, but it's better than nothing!
a secondary option if you want to persue it, add an ambulance expansion to an existing FD. it takes longer, but you can make it "not ready for action" till you want to use it, which actually you can do for any of the FD expansions
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