You can close registrations for your challenge at any time. Doing so will mean that no-one else can register for your challenge. This may be useful if you have a fixed budget and you don't want to go over this for your additional participant costs.
Here's how to close registrations:
Open Your Admin Area
Log in to your admin area (your challenge URL with "/admin" at the end).
Go to the Settings Section
Click on "Site Settings"
Click on "User Authentication"
Change the Registration Status to "Closed"
Then click "Save Settings"
Registration for your challenge is now closed. You can change this back to "Open Registration" at any time.