You might be able to block the IP-address it connects to to logon by reassigning it in the hosts-file. But that's a system wide setting, not for one user only.
And, of course, you might just rename one of the components (like a dll) so it won't run. To run it again, rename it back. That's kind of a poor man uninstall.
I am using WinXp and i want need one help. Is there any technique to block yahoo messenger withiout uninstalling it.