Vandalism is the deleting, changing or adding things in an attempt to ruin the Blue Peter Wiki. The most common types are replacing text with silly things, blanking pages or adding utter random words. Any good effort to improve the wiki, even if it's misguided or incorrect is not vandalism.
When you see that a page has been vandalised, please revert the page to the previous version. Reverting the vandalism quickly helps to protect the wiki, and it sends a message to the vandal that their efforts have been wasted.
Click the "history" tag on the page, and you'll see a list of all of the previous versions of the page.
You can see any version by clicking on the date. Click on the most recent version before the vandalism.
Click on "edit", and you'll see a regular edit page, plus a warning that says that you're editing an out-of-date revision of the page.
Hit save, and it will be like the vandalism never happened.