On the Manage Your Account page you will find a link to Change Other Values. There you will see a text field where you can place the "On Hold" message. If that field is blank, your account is active. If you put text in there, others will see that message when they try to trade or contact you. This won't stop them from reaching out to you but it will let them know you are away and it will also stop most (if not all) automatic messages that the website would normally send out.