Is it possible to allow the person submitting a webform with CiviCRM to decide whether an activity is created or now?
The following steps should do it (requires version 4.10+ of the webform-civicrm integration module):


Make the "Activity Type" field "User Select"
Customize that field to remove all options except for "None" and the activity type you want.
Optionally rename the field on the webform "Create Activity"
Optionally rename the option labels to "Yes" and "No"