There are several ways to do this.
The first is via Wear History, if for example I want to LOCK my Maitreya body I would click the lock icon.
The second way is to select Locks option from the various selections at the top of your wardrobe page and then add what you want to lock. Using my Maitreya body again I would fill out the field to match my wardrobe item inventory path after #RLV/ and then click OK.
