Let's say your event is at capacity and you need to disable people from registering to your event. Easy! Just go to your administration portal and click on Event Configuration within Event Setup. 

Now, check Close Registration. 

This will update the Register for Event button to read Registration Closed and prevent users from signing up. 

Note: If you need to change the description listed on your registration page, you can do this within the Description field in the Event Details Menu.