Special:Linksearch - find all external links to a particular site on en:Wikipedia, useful when a spam link is added by many different IP addresses or accounts.
Archive link search searches several common archives for variations on a URL that are not necessarily active links (for instance, exampleDOTcom). This is useful for finding previous conversations about particular sites.
To combat repeat offenders, you may request to have links added to the local spam blacklist (for links that have only been spammed on the English Wikipedia) or the Wikimedia Global spam blacklist (for links that have been spammed on more than one Wikimedia Foundation project).
For links generally that get used in an inappropriate way by unestablished users, but does not qualify for the meta spam blacklist or the local spam-blacklist ask User:XLinkBot to monitor it, ask at User talk:XLinkBot/RevertList.
Watch the link addition feed in #wikipedia-en-spam. There is a bot on there that reports all newly added links and keeps track of serial spammers. For more info see /IRC Channels.
If no one is around to add something to the spam blacklist, contact users at #wikipedia-spam-t on freenode.