If you want to hide your online status when visiting the forum, you can do so by modifying your profile privacy settings for the forum. When you hide your online status, regular members will not see your name listed in the Users Online / in the Last 24 Hours list, and will not see the online status in your mini profile like it normally would with your posts when you are online. Forum staff members will still be able to see when you are online, however. Your name will be seen in the Users Online / in the Last 24 Hours list followed by text that states: (Invisible) to let them know that you have chosen to keep your online status hidden.
To hide your online status, click the Profile button found in the forum's top menu bar, and then on your profile click the Edit Profile button. You'll then be taken to the Edit Profile & Settings page; once here, click on the Privacy tab. When the Privacy page loads, look for the Invisible setting and set this to Yes if you want to hide your online status from regular members. Once set, scroll down and click the Save Privacy Settings button to complete the process. Your account will be set to Invisible immediately after the settings have changed.