I heard someone calling another a Spacker the other day, which I haven't heard since I was at school. It's got a good mouthfeel, might use it myself. It could cause offence however, given that you're basically comparing them to a sufferer of cerebral palsy.
"Spacktard" takes it to the next level.
But really, when you want to properly raise your game in this regard, you gotta reach for the Shakespeare Insult Dictionary. Should be readily available in bookshops or libraries, but I'd be surprised if there isn't an online version somewhere.