Vandalism is prohibited. While editors are encouraged to warn and educate vandals, warnings are by no means necessary for an administrator to block (although administrators usually only block when multiple warnings have been issued).
Even if misguided, willfully against consensus, or disruptive, any good-faith effort to improve the encyclopedia is not vandalism. Edit warring over content is not vandalism. Careful consideration may be required to differentiate between edits that are beneficial, detrimental but well-intentioned, and vandalizing. Mislabelling good-faith edits as vandalism can be considered harmful.
Upon discovery, you should revert clearly vandalizing edits, using the undo function or an anti-vandalism tool. Then warn the vandalizing editor. Notify administrators at the vandalism noticeboard of vandalizing users who continue to vandalize despite warnings, and administrators should intervene to protect content and prevent further disruption by blocking such users from editing. When warranted, accounts whose main or only use is obvious vandalism or other forbidden activity may be blocked even without warning, usually indefinitely.
