1. Have them go to Horde settings (gear icon),
2. Hover over "Preferences" ,
3. Select "Mail" ,
4. Select "Deleting and Moving Messages" ,
5. Uncheck all the boxes and save
 
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Check the mail, mark for deletion by clicking the garbage can icon

Then it puts a line through all the emails checked.
On the right side there's an other drop down
it's labeled other

Select Purge Delete
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