Thanks, it is me who can build them, how can i get rid of my old unit designs?

Every time you patch the game, it is a good idea to clear your old units and race folder.

"My documents/My Games/Fallen Enchantress" is where mine are located
Unit designs and sovereign designs are in the folder called: "Units"
Custom factions are in the folder called: "Race"
Just delete everything in there and stuff disappears

Just in case HeavenFall's thingie doesn't work (I Don't think they disappear just by clicking retire, I usually retire my old designs to see them next time around.

Sincerely
~ Kongdej

Retire would just work for the game you're in (but if you start another game they're baack ).  Deleting them from units folder = gone for good.