Using alt+backspace key in vim command line to delete by words

try using instead <c-w> (that is ctrl+w) to erase words or <c-u> (ctrl+u) to delete lines.

http://vim.wikia.com/wiki/Map_Ctrl-Backspace_to_delete_previous_word

:imap <C-BS> <C-W>

sets ctrl backspace, i have to look at how to do alt