For once, Mr. Rewrite's going to be positive and showcase a sign that gets it right.
If you follow the AP Stylebook, go with shut-off as the noun and shut off as the verb. This is consistent with Webster's New World College Dictionary.
Well, that was a snoozer. Who comes to Mr. Rewrite looking for positive posts? He has to savage someone or risk alienating you, gentle reader ...
OK, here we go. Check how many mainstream news organizations refer to shutoff in articles indexed by Google News. Enjoy a healthy serving of schadenfreude courtesy of Mr. Rewrite. Ha, ha, ha! Look at them! Aren't we superior?!?!?
Also, Mr. Rewrite knows from painful experience that a working shut-off valve is important when one faces plumbing emergencies. Because there's no AP Stylebook entry and no entry in Webster's New World College Dictionary he will assume that the correct adjective format is a hyphenated. But Mr. Rewrite can see an argument for shutoff as an adjective and wishes AP would expand its stylebook entry to address that form.
Now get back to work.