Yes, you can revoke a Co-Hosts access to an event at any time. Just select the event in Events screen and browse to Event Details > Co-Hosts.
In Step 1 find the token you wish to revoke. You can view if the Co-Host has accpeted the Co-Host token request as well as view the access privaleges for the Co-Host. Just press the Remove Co-Host link by the token and confirm you wish to revoke access for the token.