Hi Rose!
You cannot turn Guest Registration off in Event Configuration after a guest has registered. When you go to turn it off, it will be greyed out and give you the following message:
"Guest registration must be enabled because at least one guest has registered or cancelled their registration for the event."
You can take away the option for invitees to add guests during registration by going to Website & Registration > Registration Settings > Guests > Edit > Invitees can bring guests: No > Save. This will keep the current guests as guests but no one else can be added as a guest unless you change the setting.
I hope this helps!
Best,
Bailey Good