Translations have always operated on the Honor System when in open mode; you're trusting the translators to translate the phrases accurately, and also trusting that someone who is fluent in the language will remove the Needs Review flag from things which are properly translated. Obviously, someone using Google Translate to simply fill these in should leave the Needs Review on so actual fluent speakers will be able to remove that status and/or make corrections. This has worked well for years due to the fact that there is an active translator community which actively update projects. We've never had any reported incidents, to date, of griefing or vandalization.
If this doesn't work for you, then the other option is to close down translations on your project and assign designated translators whom you trust.
No, that is not intended - the flag is there to let other translators know "I did my best, but this may not be totally accurate" so a follow-up pass can be done. Can you give me a link to a project where this is happening, and list the phrase(s) the flag cannot be removed from?
Sometimes when you revise a phrase and you are sure your revision is correct and no need to have "need review" status, but you forgot to un-check the "need review" checkbox before clicking Save button. Then the trick is to temporary revise the translation such as adding one redundant space, save it, and change the translation again but this time don't forget to un-check the "need review".