If WikiVandals are persistent, it may be necessary to complain to their ISP, but this is rarely the case.
If it seems to be needed, use the RecentPosts feature to find the IP address of someone who changed a page. You can then find out what domain name that is and other information using WhoIs (which can be optionally added to reports on WikiVandals). Complaints can usually be sent to "abuse@<domain>". Some ISPs even have toll-free numbers where we can phone to report abuse; some of us might be interested in phoning if it seems the vandal is particularly offensive.
An alternative to this approach has been suggested: FixYourWiki, take a deep breath, and let it go.
Discussion moved to WikiVandalsDiscussion.