Assertively. Take some time, calm yourself, practice in your mind what you would say to this person. We want to describe the problem to them (so there is no confusion or guessing needed), tell them how it makes you feel (using "I" statements, not being accusatory), telling them what you would like to change, and how you'll feel after the change. For example, "Hi John, I was talking to Mary today and she said that you were telling a bunch of people in the staff room that I'm the one who stinks up the bathroom all the time by not flushing. I feel really embarrassed, and upset, because it isn't true. In the future, if you think I'm adding unpleasant odors to the office, I would really appreciate it if you would come speak with me, and just ask me first if I did, and then if you would like me to change something, let me know then. I'd feel a lot better, knowing that you would approach me first before talking to others."
Stinky poo in the bathroom might not be your situation, but we can use those step to speak assertively with others. Remember to have confident body language, this is important to you, therefore it is important.