Thank you ..., I would be happy to ... but unfortunately I am unavailable. Please [....]
This shortened version takes as much thought as the 'very short' version and doesn't require a new acronym. There are cases where the longer reply would work better, but most of the time you don't need to give your reasons for saying no to someone. I'd say his response is pretty formal, and that's probably unnecessary for 95% of the unsolicited email he gets. In less formal responses you can convey some enthusiasm (real or not) with an exclamation mark, and that removes the potential rudeness ("Hi John, Thanks for your interest! Unfortunately I am not available that weekend. Best of luck, A. Guy"). Translating emotion to or from text does require a little bit of effort but it's really no different than normal spoken conversation.