Units that are Destroyed have croaked and had their body thoroughly obliterated. Once a unit is destroyed it can't be uncroaked or decrypted, due to there not being enough of its erf to reanimate or repair. When Bogroll was croaked and destroyed by Coalition forces and the Volcano Uncroaking, Wanda noted there wasn't enough of him left for her to decrypt.TBFGK 132
Some Pit traps, such as lava, have a good chance to incapacitate/croak/destroy the target. A Flying Unit failing to escape it gets sucked in and suffers massive damage, getting croaked and then destroyed.WB2014 Digdoug - Episode 18
Fire and lava seem to be necessary to destroy a unit, it's unknown if they're essential or if bodies can be destroyed by other means. Jetstone forces have begun destroying the bodies of their fallen to avoid Wanda decrypting them. King Slately's body was destroyed by being burned to avoid this fate.LIAB 93
Known Destroyed Units
A Yellow dwagon's acidic battlecrap can destroy a unit if they're completely coated.
Rather than croak, decrypted units are technically destroyed into ashes.