It is not possible that you do not know where your client list is coming from.  Except for a few sample returns, which are there by default when your account was created in case you need some practice (e.g. SAMPLE, INDIVIDUAL or BUSINESS SAMPLE), it will have only clients you created or clients of accounts to which you have access.

If you are able to make inactive a client, that must be a client you created under your own account or one that belongs to an account to which you have Master Admin rights.

In case you need this looked at, please contact Support when they open on Tue.