Timing supports blacklisting specific activities so they won't be tracked in the future. You manage the blacklist just like you would do with a regular project: To add activities to it, just drag them onto the "Blacklist" project on the left of the Timing window. You can also stop blacklisting activities by removing them from the blacklist just like you would with a regular project.
NOTE: After you moved an activity to the blacklist, it will no longer be shown in any other projects. The corresponding data that has already been tracked will however NOT be removed. To do so, click the blacklist project in the project list and delete the activities just like you would with any other project.