Sounds like a sh*t company to work for. Personally I'd probably be calling my lawyer and challenging the writeup, where I am unless its against policy, industrial relations laws or I've failed to meet an expectation outlined in my contract they can't write me up for it.. That and its a way of documenting management BS in case there turns out to be an issue where you'd have to claim unfair dismissal.
Personally, I wouldn't quit, I'd look for a job elsewhere & once I'd secured one I would be having a fairly frank chat saying that I wasn't happy with how this was handled & I simply didn't want to work there anymore. I'd be polite & professional about it (offer a decent notice period, etc) - never burn bridges. Unless you're in a bad state financially or can't get a job elsewhere (you're in sales & you met your targets, surely you can get work elsewhere..) I wouldn't sit idly and cop a slap to the face like this.
FWIW, My main thoughts behind leaving is it sounds like you're not happy there - if you love the other 99% of the job and just feel like this is a rare once off, I'd probably still sit down with my manager, tell them that I didn't think this was a fair writeup and ask them to provide me with more feedback so if there is an area where i'm lacking, I can improve it.