Can my employer decide to fire me although I have already resigned?

Yes, an employer can fire you while you are working a notice if you violate company policy or behave in any other inappropriate manner. Slander is a legal term that does not necessarily have anything to do with employment law. As a separate issue if you can prove that they have communicated fraudulent information to another party then maybe you could sue them for that (I'm not an attorney so please verify), but it is not relevant to the first question.