Per the official rules, https://serverfault.com/privileges/protect-questions

> **Which questions can be protected?**
>
>Any question, even a deleted question, can be protected and unprotected, subject to the following restrictions:
>
> -   The question must be at least a day old
> -  A user can unprotect a question protected by him or herself, but not one that has been protected by someone else

So you have to give the spammers a day's head-start before protecting.