Join Codes - need to deactivate

HOW HARD IS THIS?! This should be a simple fix that is easily taken care of. Reposting -

Some of my little darlings cannot restrain themselves from joining my class every time we use KA instead of logging in using their already-existing account. Teachers need to be able to turn off/deactivate join codes, or be able to require teacher approval for a new student to join -- especially after the beginning of the year when classes are set.

This is also a security issue, as anyone anywhere who gets ahold of a join code could join a class even if they have no business there.

How can I turn off the join code, or require me to accept new students before they can join?