In ePO 4.6 I can set the custom 1-4 properties and description fields for a given systems using the 'set description' button under the agent menu. This same functionality is available through the web api using the system.setUserProperties method.
All is working fine until I try to set one of the values back to empty (null). I've been throught the docs, KB and web searches but haven't been able to find a way to clear out the data.
This is a known bug. The fix should be released in 4.6.3 and you'll be able to clear out the values properly in the UI action as well as in the server task.