When viewing the administration area of an Activity or Pathway (see find content in Find Learning or Content Manager if you're unsure how to do this), click on the Discussions tab on the left-hand side.
The Expert(s) assigned will be listed under the Users heading.
Click on the X next to the User(s) you would like to remove.
The Expert(s) will be removed.
Note: They will not be notified automatically by email so you may wish to communicate this change directly.
Click on the Save button at the bottom of the page.