With the latest UCI update there are shown additional options in the timeline like "Assign", "Delete", "Close Activity", "Add to Queue" and "Open Entity Record". Currently there is no other possibility than to withdraw the user rights or use unsupported java script to hide these options. As in Views and Forms there should be a possibility to configure these options (like you can do with RibbonWorkbench). For example to hide the 'Close Activity' option for specific role - these users can still read and edit an activity but aren't able to complete them, because this may should only be done by another user group.