The stop accepting calls button can be found within the call log towards the top right below the Settings button.
Clicking this will prompt the agent to stop accepting calls. Inbound calls will still arrive and be charged but the agents who have this enabled will not receive calls. This button will flash when an agent has it selected to stop accepting calls.
Finally, within Settings > Manage Users if an agent is not accepting calls their status will show as two red symbols. In the example below John Doe has pressed the Stop Accepting Calls button.
To start accepting calls again press the Stop Accepting Calls button.