Steps Involved: 
- Open SharePoint 2010 Management Shell by going to Start | All Programs | SharePoint | Microsoft SharePoint 2010 Products | SharePoint 2010 Management Shell (Run as Administrator). 
- Run the following script.
Powershell Script:
|  ##Delete a social tag associated with the current user for the specified URL and social tag keyword using SharePoint 2010 web service in powershell
 $uri="http://serverName:10736/sites/ECT/_vti_bin/SocialDataService.asmx?wsdl"
 ## $url is a string that contains the URL for which the all the tags has to be deleted for the current user
 $url="http://serverName/sites/teamsite/"
 $keyword="new tag"
 
 ## Get the credential for whom the tags has to be deleted
 $credential=Get-Credential
 
 ## Web Service Reference - http://Site/_vti_bin/SocialDataService.asmx
 $socialDataServiceWebServiceReference = New-WebServiceProxy -Uri $uri -Credential $credential
 $socialDataServiceWebServiceReference.DeleteTagByKeyword($url,$keyword)
 | 
The above script will delete the tag named "new tag" for the current user and specified URL.