You can't really disable the context menu to begin with. [You can only construct fragile barricades to keep people from invoking it][1]. But the fact that this is an external iframe only compounds the issue. No, you can't keep the users from activating the context menu on your iframe. Sorry.
[1]: https://stackoverflow.com/questions/381795/how-to-disable-right-click-context-menu-in-javascript
You can't really disable the context menu to begin with. [You can only construct fragile barricades to keep people from invoking it][1]. But the fact that this is an external iframe only compounds the issue. No, you can't keep the users from activating the context menu on your iframe. Sorry.
[1]: https://stackoverflow.com/questions/381795/how-to-disable-right-click-context-menu-in-javascript