It seems there is some problem with theme setting and maybe the versions of multiselect plugin. Please post a code for your problem in order to get idea what is happen.
To be a honest I do not perform these tests.
As far as I read the authors comments it seems that the only way to see this is to try to implement this approach.
The buttons have id. You can either show/hide them by condition or add to them a class ui-state-disabled, which disabled the action too. See the Oleg’s post here
