In P3D menu go to Options - Controls... on the left side select axis assignments, then at the top select Flight Mode and Slew from the drop down menu. Make sure there are no hardware controllers assigned there. If there are, delete them. If using FSUIPC for all your controllers and all their assignments including slew, you can just go to Other Controls in the menu and under Controllers, uncheck the Enable Controller(s) box. That will solve your problem too.