There are several ways to block access to participants.
Set password for space
If you set a password for your space, then no one, even having a link to it, will be able to enter any room. Here's how to set a password like this:
Set your password and click "Lock Space"
If there are already other participants in your space, they will not be kicked out of the space, and nothing will change for them. They will only need to enter the password when re-entering the space.
Set a countdown timer
If you set such a timer, then no one except administrators will be able to enter your space until the specified time.
Here's how to do it:
After you set the date and time and click "OK", all participants in the space will be kicked out to the welcome page and will see the following message:
And the next time they try to enter, they will see a countdown timer
You may use it to end the event. Set a new start time and everyone (except admins) will be kicked out of space.
At the same time, administrators will have the opportunity to enter the space by clicking on the button under the timer