How can I delete user-defined cell formats?

[Zilk]

When I format a cell using a custom format (for example, prepending the text ‘old amount:’ to a number), it sometimes takes several tries until I get it right. The incorrect, and now unused, formats accumulate in the list of user-defined formats and I can’t find a way to remove them. They appear to be stored on a per-document basis.

Other than editing the raw XML files by hand, how can I get rid of unwanted cell formats?

[mariosv]

With [F11] (Menu/Format/Styles and Formatting you get the window with styles, right-click and deleted on the style to eliminate.

[Zilk]

That only shows styles, not formats. What I’m talking about is this: right click cell A1, set a new custom number format, hit OK, then Ctrl+Z (undo), and you’re left with an unused number format that can’t be deleted (AFAIK).