I'm posting on this old post mainly for completeness and to help anyone else out that has this same issue.

I'm not sure how far back this goes, but I know it works in SQL Server 2008 r2.

the xp_cmdshell command has an optional parameter to specify no output. In your example here your sp would have a line like this:

exec xp_cmdshell @cmd, no_ouput

BadOed