You can stop a user synchronization tasks from the user directory connector association page.
Do the following:
Open the QMC: https://<QPS server name>/qmc
Select User directory connectors on the QMC start page or from the Start drop-down menu to display the overview.
Select the user directory connector that you want to start a task for and click Edit in the action bar.
Select Tasks under Associated items.
The User synchronization tasks overview is displayed.
Select the tasks that you want to stop and click Stop in the action bar.
x out of x items were successfully instructed to stop is displayed at the bottom of the page.