Is this possible? I have a customer who doesn't want his users to be able to select the blank field, but whose workflow process requires him adding/removing questions about intermediate steps.
Is Workflow supposed to be blanking out the field or the user? There is no way for the user to set it to blank through the UI if blank is not in the list (well, for independent fields, you can remove it from the doc altogether). But if you leave it blank in Workflow, it will set it to that value even when blank is not in the list.
BTW - for any one stumbling across this post.
If you have a one of your list choices as default you'll notice you can't delete it out, you can only choose other things in the drop down list.
If you click the > button and choose token editor you can erase it. This took me a few minutes to figure out once I hit one of those.
You should just be able to go into the template and configure the security rights so that only the person needing that field can even see or edit it
EDIT: Try using the following link to help guide you through the configuration.
It's not a question of who can see or edit the field.
We add a field asking if an external task has been completed. Once that task has been completed we remove that field. However the only way I've been able to remove it is to add a blank to the list.
If I have a blank on the list, occasionally a user will choose that blank option. If they do the process is now messed up because that field is now removed from the document. There are a large number of these fields that get added/removed and the customer doesn't want to add them to the templates, and doesn't want new templates for 20 different one step questions.
And I can't use the "remove fields from unassigned templates" because sometimes there is more than one active at a time.
Is Workflow supposed to be blanking out the field or the user? There is no way for the user to set it to blank through the UI if blank is not in the list (well, for independent fields, you can remove it from the doc altogether). But if you leave it blank in Workflow, it will set it to that value even when blank is not in the list.
Hahaha 
- I see what the problem is now!
When I went in and changed the field to remove the blank and tried to republish the workflow in designer, I had an error that said ' ' is not a valid list value.
However that was there because when the workflow was designed the "add a blank to the list" option was checked and that's what was selected when the workflow was designed.
Going back in and removing the single space took care of all of those!