Simple answer - You don't.
You're going against the first rule of interface design - 'Do not confuse the user'
A checkbox has a certain expected functionality. The user would expect to be able to select one or more checkboxes.
A radio button has a different expected functionality. You may select one and only one.
Changing those around would lead a user to think something is broken, because every other checkbox he has ever encountered works the same way, except yours.
If you want radio button functionality (one and only one selection), then use a radio button.