Well firstly their craft is written off, componants used for our services.(Army).They nare not suitable for our own transportation, but any technology has always been taken. The ocupants are medically examined, usually disectesd and preserved. Nowadays, they are studied psychological in experimental psychology units, similar to mental institutions.In short they are used for scientific study, and militry technology/hardware.
They nhave to be destroyed .They can never recover the required pychology to return. The self same psychology and nature that they come with , they cannot recover it.They are stranded and destroyed.(The occupants, after the spare parts and usefullness has finished.Because they are foremost creatures of pure mind, psuchology is the main problem.THe key here is the psychology involved with their capture.this is not unique to them, it applies to all creature, an us.Lock and key, basically.
Like any prisonner will tell yoy, one has to fulfill the criteria requred by the captors, to make effective the removal of the lock and key.