You can edit the settings for your client so when their claims are created insurance will know to reimburse the client and not the provider.
Go to the client's profile > Edit Client Info > Billing and Insurance > Insurance Claim/CMS Fields > check the box Enable Additional Fields > Accept Assignment? > click the bubble for No.
Now when you create claims for this client in SimplePractice Box 13 will not have the Signature on File box checked.
And Box 27 will have Accept Assignment? No.