Deleting a timetrack is relatively easy. Click on the 'Effort' link in the project detail screen or the amount next to the 'Volume done' field in the task detail screen to get the list of entered timetracks. Next click on the relevant number in the '#' column and click on the 'delete; menu option in the upper left part of the menu. Confirm the confirmation message that pops up (see next picture) and your done.
The login screen