Your role as PM is to ensure you have the correct staff who will be capable to deliver the project on time and budget with the acceptable quality.
the project sponsor should have the same goal and this should be the common ground where you will resolve this conflict, you need to listen to the perspective of the sponsor and understand why he thinks it is necessary to include such team member in the project team, may be he has a point and can convince you. Otherwise, you will have to tell him your concerns on this team member and the consequences that could happen if he insist, hopefully he will agree with your point of view.
