Proving slander? advice please?

someone is claiming I spread a dirty rumor at my university. I did not do this but I have had inappropriate behavior in the past so I can see how I am a likely suspect. I cannot provide reason why the person who reported this would even target me& make this up, making me look even more suspicious. It's quite serious and I could face charges of slander as well as expulsion from the school I hear if they continue to believe it was me. Is there ANY way to prove I did not spread this dirty rumor? Advice please?