Clearly you need to stop them spamming in the first case (needs longer answer about Interceptors etc).
But if you've already got the damage - can you just delete the spammer accounts and this should delete their content as well. Admin Console doesn't declare that you will also get their bookmarks but it seems that they do go when you delete the spammer account.
Thanks for your response. Actually, even when I delete the user their bookmarked tags don't go away. In addition, it seems that I can't identify WHO the user actually is when you click on the bookmarked link/tag. If you take a look at the screenshot, it doesn't provide me with any info at all other than "a user" has bookmarked a spammy link and tagged it inappropriately. Any advice on how to find which user is doing this and blocking them?