You can't. We are monitored by regulators and cannot delete a system completely from our records.
You can Reset your strategy (for a fee) and obviously have a fresh start or you can make the strategy Private which means the strategy will no longer be visible to the public (but it still takes a spot in your plan).
The Reset Strategy menu is is next to the Subscribers menu while the Private button is under your name in the upper right corner (slide the bar from Public to Private).
If you plan to manage a new strategy to replace an old one, you must choose the Reset option.