Are there tables that are truly only in specific clients at the DB level? I think the only thing that distinguishes one client from another at the table level is the MANDT field. So, I don't think you'd be able to truncate the table entirely, or you would lose the data from other clients in it, unless you can be certain that there are no rows in the table for any other client. That could be exhaustive to determine, although the effort may be worthwhile for the top x largest tables.
Mohammad, another possibility, instead of using TRUNCATE TABLE at the SQL Server level, would be use DELETE <table> WHERE MANDT = <client to be deleted>. This would not be nearly as fast as a TRUNCATE, but you might consider putting the database in SIMPLE recovery mode, then using the DELETE, table by table for your largest tables, then returning the database to FULL recovery mode and taking a new full backup.
I'm not sure I would actually recommend this process, but as this is a test system, you can certainly give it a try. The worst-case scenario is that you have to restore it again from your original source copy.
Is there a reason you can't just use the same client number in the target system as in the source system?
Regards,
Matt