I had a guest post on my site with out registering, and wheni clicked on there name it would not let me.under the name it said guest how can they post with out being a member and how can i change that so they can not do that
This should be in the support area, but it's a permission thing.
Check from admin -> permissions -> guests (modify)
There you can set what is allowed for guests on your forum and what is not.
It could also be a member post. If a member makes a post and their account is later deleted, their posts will appear as guest posts. To see if this is happening, turn on administrator approval when deleting member accounts.