Well is a "Go Around:" spoken on the R/T by a Controller an ATC instruction ? Yes it is..
Most national AIPs follow Doc 4444 where it says that in IFR all ATC instructions have to be followed by the crews . I guess it is the same in Canada or in the US.
PIC can of course always decide not to follow an ATC instruction , but he/she will have to explain why afterwards., and if it endangered someone or other aircraft you will be in real trouble.
The reply "unable" comes to mind of course but not really applicable to a Go around I would say.