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.