On Customize dashboard can't disable the Management block

I tried to customize my Dashboard at [myURL]/#overlay=admin/dashboard .
I've turn it on my Management block: drag'n'drop to the #dashboard_main (in Garland theme).
After that I tried to remove it by drag'n'drop back to the #disabled-blocks, but that was impossible anyhow.

(The Customize dasbord works perfectly with the other "core" blocks.)