The technique should work even if you find yourself thinking about the task at other times; it just might not work as well, because of the effect that jimrandmoh mentioned about reminders reducing your inclination to do something. A variation of the workaround I mentioned for dealing with others works to mitigate the effect of self-reminders, though - don't just tell yourself 'not right now', tell yourself 'not right now, but at [time/event]'.

I can't say much about how to disable involuntary self-reminders altogether, unfortunately. I don't experience them, and if I ever did, it was long enough ago that I've forgotten both that I did and how I stopped. I have, however, read in several different places that using a reliable reminder system (whether one like I'm suggesting, or something more formal like a written or typed list, or whatever) tends to make them eventually stop happening without any particular effort, as the relevant brain-bits learn that the reliable system is in fact reliable, which seems quite plausible to me.