+1

@jostster ,
for relationships you can use

ActiveAdmin.register Order do
  filter :user_id_not_eq, label: 'Except User', as: :select, collection: ->{ User.all }
end

string search is still possible

ActiveAdmin.register User do
   filter :username, filters: [:not_start,  :not_end, :not_cont]
end

one issue is there is no aliases to use it like not_starts_with, not_ends_with ... and not translations for this.
I think providing aliases and documenting it to close the issue.
@varyonic , what do you think ?

Using version 1.0.0.pre of ActiveAdmin, I was able to implement 'Does Not Contain' to a string filter with a few small workarounds, guided mostly by @Fivell's last comment.

ActiveAdmin.register User do
  filter :username, as: :string, filters: [:contains, :equals, :starts_with, :ends_with, :not_contains]
end

Note that in order to include the default options in the drop-down, they needed to be included, and :not_cont needed to be changed to :not_contains.

In my en.yml translations file, I needed to add a translation for :not_contains:

en:
  active_admin:
    filters:
      predicates:
        not_contains: "Does Not Contain"

With these in place, filtering by 'Does Not Contain' works as expected.

Edit: This does not work as expected. It will simply return no results.

@ryanflach I managed to get this working by using :not_cont and adding the translation for not_cont

@ghilead and @ryanflach thanks guys it works like a charm. It would be nice to add these across the board for any string.

This should really be a PR, such a great feature.