no, they can't..once visa is issued, not even you, the applicant, can't cancel that..and RPRF paid will not be refunded if visa is stamped already..sponsors play part during your PNP application, but in visa application, it will be your part...although it will be difficult for you to establish upon landing to Canada without your sponsor's help... and if sponsors deliberately try to cancel your visa, well, they can try, but it will have negative feedback on them and will probably have problems in their future sponsorship..