You can report obvious and persistent vandals at Wikipedia:Administrator intervention against vandalism (WP:AIV) to be blocked from further editing. Although blocks may be issued at any time, depending on circumstances, typically, before seeking a block there, a final warning in an escalating series should have been posted to the user's talk page (for example {{Uw-vandalism4}}, {{Uw-spam4}} or {{Uw-speedy4}}), and the user must have vandalized within the last few hours, including after the final warning was given. Various warning templates can be found at Wikipedia:Template index/User talk namespace (easily remembered by the shortcut WP:WARN). Your block request is unlikely to be acted upon unless you follow these steps. Cases that are not simple vandalism can be reported at Wikipedia:Administrators' noticeboard/Incidents. Of course, in conjunction with warning against and reporting vandalism, you have the ability, mandate and are encouraged to revert all instances of vandalism you find yourself.