Avoiding automatic creation of civi contact upon drupal account creation
There is no way via the standard config to do this afaik. In theory, if the drupal contact that is created matches a unique email for a civi Individual (that has no drupal user yet) then the match should be made.
Otherwise you have to drive that type of clean up via your dedupe workflow.
Also just spotted you said Organisation. So in theory, again, Drupal users can only be Individuals - since organisations find it hard to use the keyboard. Perhaps that is part of the issue you are needing to resolve?