Devirtualization is the process of deleting an entity from the a virtual world. It seems that getting devirtualized can be pretty painful. There are several ways to induce it. If there is a compatible program available, whenever something is devirtualized, it is materialized.
In the episode Spectromania, Jeremie explained that after being devirtualized, they cannot be virtualized again for twelve hours. Although, in many cases during the original series, the Lyoko Warriors have been seen traveling to and from Lyoko in a span of less than 12 hours.
Means of Devirtualization
The entity loses all of their life points and the virtual body is discarded. The Lyoko Warriors and Monsters can lose life points by either being hit by being attacked, smashed by a large object, or by falling from great heights. The entity seems to vanish, in whatever procedure applies to the type of entity in question "see below". If a compatible program is available, the entity then materializes through the Scanner again.
Occasionally, a character has nearly fallen into the digital sea, only for another warrior to save them by using friendly fire, to induce this. This has happened frequently enough that Jeremie commented that he needs to boost the devirtualization program.
If a materializable entity is on a virtual world, and an RTTP is run, the entity will return to wherever they were on Earth at the time. If the entity can not be materialized, due to being a monster, being a sector, being pre-season 3 Aelita, etc; then they will remain on the virtual world in question, as if nothing happened.
There are several programs to move a warrior back from Lyoko to the real world. The operator (usually Jeremie or Aelita) is able to execute a series of commands to bring a given character back to the real world. Essentially, these programs are the reverse of the Virtualization program. Aelita's Materialization Program was one of these.
Different types of entities have different procedures for devirtualization.
X.A.N.A. William: Would disappear in a cloud of red and black smoke.
Ninjas: The same as the Lyoko Warriors, but the pixels turn green.
Monsters (except for Guardians and Kolossus): Seasons 1-4: The monster explodes into colored geometric objects, and the objects flicker away. For monsters who can fly, they blow into pieces and the remnants of their bodies disappear.
Monsters: Code Lyoko Evolution: They explode as red blocks, and then implode, with a similar effect to the circular laser. For a real life phenomenon that looks similar, observe a star undergo supernova, then collapse into a black hole.
Guardians: They explode.
A Sector: the surface turns invisible and the vector lines disappear.