Scenario:
You have been trying to start up the User Profile Synchronization Service and it has been in a starting state for hours/days/weeks.
Resolution:
Run following commands in the SharePoint 2010 Management Shell to stop the service:
get-spserviceinstance > driveletter:outputfilelocation.txt
After the above command review your output file and you will see a list of services that are running in SharePoint that looks like this: 
Find out the service ID for user profile synchronization service and then use this command to stop the service:
stop-spserviceinstance <ServiceID> 
In PowerShell you will be prompted if you want to perform the action, to which you reply “y” and the service becomes unprovisioned and stops.
Update:
I have used this same method 3 times in the past week to stop different SharePoint 2010 services for various reasons. Worked like a charm every time.