Please do not do that, it is very annoying.
The right menu is there for a reason, and it should be left there. Many browser extensions add entries to the right click menu and the user should be able to use it in any page he visits.
Moreover you can use all of the functionality of the right click menu in other ways (keyboard shortcuts, browser menu etc etc etc) so blocking the right click menu has the only effect of annoying the user.
PS: If really you cannot resist the urge to block it at least do not put a popup saying "no right click allowed".