Please try to put as much effort in to deleting and tidying your old stuff as you do adding new stuff: FixYourWiki!
There is no security. Anyone can delete content. Some stuff might get restored. Don't fret about it; it's part of WhyWikiWorks. Also see WhyNobodyDeletesWiki.
Please read DeletionConventions before deleting pages.
To delete a page, replace its text with the word "delete". If you are the sole editor, the page is deleted immediately, otherwise a second editor must confirm the deletion to remove the page.
A deletion can be restored using the EditCopy or, if already seconded, using the HistoryPages (http://c2.com/wiki/history). However, note that history pages are disposed of at the end of each month.
There are tips and discussion related to the longer-term evolution of a wiki at OneYearWiki.
OneMinuteWiki, OneDayWiki, OneMonthWiki, OneYearWiki