You can exclude subclients from the SLA calculation in the SLA Report.
Go to any of the related reports from the SLA Client Summary chart:
Click Met SLA.
Click Missed SLA.
Optional: Go to any of the related reports from the Missed SLA chart:
Click Failed SLA.
Click Missed SLA with No Jobs.
Click Missed SLA with No Schedule.
For each subclient you want to exclude, beside the Client name, select the box, and then at the top of the table, click Exclude from SLA.