Set tax lookup On/Off - By default the Tax accounts lookup will be displayed - when the cursor is in the Tax field of the Document line on the Document entry screen. To deactivate this, select (tick) the “Tax Read-only” option.

To change the setting, you need to tick the "Readonly tax" option in the Documents tab (Setup → Access control on the Setup ribbon).

Only when you re-open the Set of Books, and process documents, the Tax account lookup will not be displayed when processing documents.