Wild subscriptions

Hello, I own the http://romaframework.xwiki.com/xwiki/bin/view/Main/WebHome space. Every day, since about one month, I register about 5 new subscriptions. Most of these subscriptions are spam or simply advertising about Cialis, Viagra, etc. How can I protect myself by wild subscription? Is there an approval system to trust users? Thanks a lot in advance! -- bye, Luca Garulli Blogging on: http://zion-city.blogspot.com http://www.AssetData.it http://www.RomaFramework.org - The new way to build applications http://www.OrienTechnologies.com - Light ODBMS, All in one JDO solution