Use the following commmand line :
stsadm.exe -o setproperty -pn [days-to-show-new-tag] -pv 0 -url [virtual-server-url]
by replacing :
[days-to-show-new-tag] with the number of days to show the !New tag (set to 0 to disable)
[virtual-server-url] with the virtual server url (http://… )
More informations on Microsoft knowledge base url : How to stop the !New tag appearing when you add items to your SharePoint Team Services and SharePoint Services Web site
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