Perhaps a check box on the Account Settings page.
Search already automatically excludes delisted securities unless chosen from the drop down menu.
I've no idea what percentage of players also dabble in .cx securities, but I know I have no interest in them. It can get annoying sometimes when you enter a Ticker Symbol in the window, and instead of going straight to the stock page it goes to another page where you have to choose.
It's not really a big deal, but I think it's one of those little things that could make HSX just a little bit better for a lot of users.
Cheers.