You can delete a skill by going to your skill page, clicking the edit button at the bottom of a skill and then clicking the skill tab. At the bottom of the page you'll find the Delete skill button.
Alternatively, you can also make your your skill inactive. This makes it no longer visible and disables the ability of others to book you. This way you can always continue where you left off whenever you want to.
Note that you can only delete a skill if there aren't any pending or confirmed bookings for it. If this is the case, you could decline / cancel them before deleting the skill.
Important: any data associated with a deleted skill will no longer show up in your statistics.