The best way to prevent the prefix [PREVIEW] from deploying in live sends is to wrap the subject line with AMP script that checks if the send is a test send or a live send. For example:
%%[
IF _IsTestSend == true THEN
SET @subject = "[PREVIEW] " + @subject
ENDIF
]%%
Using Proof instead of [PREVIEW] does not solve the problem, as it still shows up in live sends. Adding
[PREVIEW] to the subject line validation list does not prevent it from deploying, but rather prevents any email with [PREVIEW] in the subject line from being saved. Requiring several campaign approvals does not guarantee that the prefix will be removed before sending.