It is possible to prevent people from registering themselves on your Wiki. The code below assumes your Wiki's URL is http://mywiki.xwiki.com/.
Go to http://mywiki.xwiki.com/xwiki/bin/view/XWiki/RegisterNewUser and protect this page under the More Actions menu. For example, make it so that only you can view it by granting yourself admin, view, edit, comment.
Two important cautions:
- Make sure you protect just the page, not the Space or the whole Wiki.
- There is a bug in the current Wiki software version that causes the RegisterNewUser page to cease functioning after you change it in any way -- even if you just protect the page. You will need to email Support to have them reset the page's functionality after you protect it.