delete_site capability. How does it work?

In subsites under tools there is a menu option called "delete_site". I want to remove the capability of subsite owners direct access this so using menu editor pro I have had a look at it. The capability required for this is "delete_site". However there is no "delete_site" capability. I registered this capability myself and made sure subsite owner account did not have it assigned but this menu option is still available.

I imagine that this is a dynamic capability of sorts that is worked out based on who created site in first place but I cannot find any details on it or how to override it.

Anyone know how this works or how to edit it?