By default, a notification is sent to the Slack channel when a task is completed. If you don't want the task completion notification in your main Slack channels, you can turn that setting off.
From Workast, click on the space.
Click on three dots here:
Click on Settings
Click on the button to change it from on (red) to white/grey (off.) Click "Save" to update your changes.
For information on customizing or turning off the notifications in your Workast channel, click here.