You can unassign or unlink external resources from a user, from the General tab as with any other resource. See Creating Users and Working with User Accounts for instructions.
Unassigning or unlinking an external resource from a user does not create a provisioning request or a work item. When you unassign or unlink an external resource, Oracle Waveset does not deprovision or delete the resource account, so there is nothing for you to do.
You can use the Delete Resource Accounts page to unassign or unlink resource accounts when the Delete operation has been disabled.