In cases of vandalism and spam, administrators (there are more than a thousand of them at the English Wikipedia) can take two types of preventive measures unavailable to normal editors—protecting pages and blocking vandals.
If a page is repeatedly vandalized by a changing cast of anonymous IP editors, then temporary semi-protection of the page is probably appropriate. Semi-protection means that registered editors can still edit the page (starting 4 days after they register), but anonymous IP addresses can’t. (Anonymous users with suggestions for changes should post them to the article talk page.) It’s quite unusual to fully protect a page because of vandalism; full protection is normally done only for cases of major content disputes (see Chapter 10).
In the first sentence of the prior paragraph, two key words are “repeatedly” and “changing.” You shouldn’t request semi-protection of a page unless there have been at least a half-dozen vandalizing IP edits in the last 24 hours or so. If there are fewer, administrators may feel that it’s better to simply manually revert the vandalism. And the IP addresses need to vary or otherwise be unblockable. Otherwise, administrators prefer blocking a few IP addresses.
To request semi-protection, post at Wikipedia:Requests for page protection (shortcut: WP:RFPP).
Note that semi-protection is normally only temporary: The goal is to get vandals to lose interest in the protected page. Anonymous contributions ...