Is it possible to restrict Journey Builder to not have access to all Salesforce objects via the Entry data?
Is it possible to have read only access to a Salesforce object in Entry data but prevent any updates to the fields of the object via activities?
There are some granular permissions regarding how has access to these on the journey canvas, outlined in this help doc.
Some Marketing Cloud customers want their users to access Salesforce events but not Sales and Service Cloud activities. Some want the opposite experience. Two new Journey Builder permissions in Setup let you grant access to these features separately.
To restrict access by user to objects, you would need to look at "scope by user", but have not tested it with JB salesforce events
Scope by User This option configures data access on the Marketing Cloud Connect user initiating the send. Reports and campaigns that are visible to the user are available for selection as a target audience for sending. The list of subscribers returned is limited to what the user initiating the send has access to view.