OGNL Presentation - Mark as Read-only?

Hello,

In the presentation portion of a UserInteraction object, I have a default value of "#_userdisplayname".

I use this to capture who the current user that is answering the interaction (approval process). 

Is there a way to populate that with the current user (non-editable)?  I use this for auditing purposes but do not want the person approving the action to be able to change it.

BTW.....

The end-user is using the web client in 6.x to answer these actions.

Thanks

B