Unfortunately, rumors are something that can’t be resolved. You can try to clear the rumor but it is impossible to change everyone’s mind. As you said, your senior proposed you to be her boyfriend and when you rejected her then she spread this false rumor. There are people like your senior who can’t take rejection and defame others. I would suggest you not to panic and handle this situation carefully. Even if you will talk to your senior about this matter but the rumors have already been spread. So, I would suggest you to give this matter some time, and eventually, this matter will die down like others.