My father has passed away he named myself and his girl friend beneficiaries of his pension. They aren''t legally married can I stop her from receiving

Whether or not she can receive a portion of the pension depends on the administrator of the pension.  Is your mother still alive?  If she is, were she and your father divorced before death?  This could all impact on whether or not she can receive the pension.  No matter what, you should contact the pension board and advise them that they were not married, she may have represented herself as being married to him.