I must say that beyond the actual commandlets to retract (more or less obvious), you need to asses what the feature is actually doing.
Simple answer - deactivate the feature via comandlets - but not before considering implications of that feature - what was it doing?
- Does your feature deploy Page Layouts (which might be in use ?)? Be aware that removing it would not Unlock Page layouts in use - so you would need to reset to other page layouts, and only than really clean up.
- Does it perform any updates on configuration files - you need to Undo those changes
- Does the feature deploy and attach any Event Handlers - you need to de-attach those or you shall get yourself a lot of errors from missing the actual assembly, class, etc.
- Does your feature deploy any Site Definitions - even more trickier if used already.
These are just a few, are passing through my mind at a glance, which you should consider, beyond the actual removal via dedicated commandlets. Also, you would need sometimes to remove it physically from the FEATURE folder - from all Web Front Ends