Close off is an idiom used to describe the action of blocking or sealing an entrance, pathway or communication line to prevent entry or exit. The antonyms of close off are open, allow, unseal, unclog, free or clear. The opposite of closing off an entrance, opening it up could create a sense of welcome or invitation. Allowing a pathway, particularly in communication lines, could lead to opening up opportunities and mutual benefit. Unsealing a door could mean granting access or giving permission to enter. In a broader sense, clearing or unclogging could lead to the unrestricted flow of ideas and information.