Webform CiviCRM activity - prevent email notification
CiviCRM Core will (by default) send out an E-mail to Contacts who are assigned to an Activity. 

You can change this setting to Not send out any E-mail to Contacts who are assigned to an Activity and handle Email notifications on a per webform level. If you need an example of that - let me know.

Here is an example:
On the form: E-mail + a checkbox (without label: &nbsp; and text: Send E-mail Notification:



Conditionals -> if the checkbox is checked -> show the E-mail field:



In Webform -> E-mails -> send E-mail to Component -> E-mail:



Of course you can make this as complicated as you want (also e.g. by including Activity Status and/or Activity Type in the Conditionals.