How do I lock or cancel a time slot?

Go to Sign-Up.

Go to Sign-Up.

Select the Sign-Up tool from the Tool Menu of your site.

Click on a meeting title.

Click on the time slot you want to lock or cancel.

Click it in the time slot table and use the options presented underneath it.  Note that you can first move participants to another time slot using the edit buttons beside individual entries.

Once locked/canceled, the time slot will have an icon indicating that it has been locked or canceled.