In Newyork you can not make the catalog variables readonly in agent workspace.

How ever this feature is present in Orlando release by using g_form.setVariablesReadOnly(isReadOnly) API
This is the document for that:

https://docs.servicenow.com/bundle/orlando-it-service-management/page/product/service-catalog-management/concept/service-catalog-variable-editor.html

You can completely remove the catalog variable from the agent workspace.
This is the process for that:

You can go to the RITM record in the platform, configure the form layout for 'Workspace' view and remove Variable Editor field from the form.
It would then no longer show this in Agent workspace.