Hello,
In SEPM 11.x, it is not possible to reduce the Virus definition size to keep for a single host.
I would suggest you to migrate to the SEP 12.1,where you would be able to reduce the Virus Definition limit to 1.
Check this Thread: https://www-secure.symantec.com/connect/forums/virusdefs-folder-size
By Default, Symantec Endpoint Protection Manager downloads and keeps 3 Revisions of Virus definition versions in its Repository.
Configure the Disk space Management for Virus Def.
To configure the disk space used for LiveUpdate downloads
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In the console, click .
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Click and select the site that you want to configure.
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Under Tasks, click , and then click .
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Under Disk Space Management for Downloads, type the number of content downloads that you want to store.
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If you want to reduce the amount of disk space used, uncheck the option. Note: Disabling this option also disables the ability of Symantec Endpoint Protection Manager to construct deltas between content revisions and may adversely affect network performance for updates.
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Click .
Disk Space Management procedures for the Symantec Endpoint Protection Manager
http://www.symantec.com/docs/TECH96214
Drive Space used by Virus Definitions Updates
http://www.symantec.com/docs/TECH141811
Hope that helps!!