Blocking a client is a soft stop – they can still create messages, but nothing gets sent. Instead, their messages wait in the SMS queue until you unblock them. This is useful when you need to pause a client's traffic without losing any messages they've already submitted.
Go to Profiles → Clients.
In the Actions column, click the block icon next to the client.

Blue icon – the client is active. Click it to block.
Red icon – the client is blocked. Click it to unblock.
A confirmation message will appear at the top of the page with the client's name and their new status.
While a client is blocked, their messages queue up in Current Traffic → Queue of SMS and will be sent automatically once you unblock them.