These Ruling(s) have not been confirmed as applicable to the TCG.[Notes 1]
In place of declaring a FULL card name, a precise description of a card's stats, appearance, and/or effects, is considered acceptable so long as both players are in clear agreement over which card is being described.
In the case of a card like "Luster Dragon", since the original printing of "Luster Dragon #2" did not have the #2 by its card name, you should ask the player for additional information. If they say "The Level 6 one" is then enough information provided so that both players know which card it is.
"Cannot play" ("cannot be used") includes Normal Summoning/Special Summoning the declared Monster, activating the declared Monster's effect, activating the declared Spell/Trap Card, changing the declared Monster's battle position, or attacking with the declared Monster.
These TCG rulings were issued by Upper Deck Entertainment and have since been deemed unofficial by Konami. They were previously considered official and can still be correct, as long as they follow the official gameplay rules, or unless Konami has issued a ruling that says otherwise. References: 
"cannot be used" means that the declared card cannot be placed onto the field from your hand. A declared Monster Card cannot be Summoned or Set from your hand, and any effect it activates while in your hand cannot be activated. A declared Spell or Trap Card cannot be activated or Set from your hand. Every player in the Duel cannot "use" the declared card.
The declared card may be used to pay costs. The declared card can be discarded, Tributed, or sent to the Graveyard.
A declared Monster Card can be Special Summoned from the Deck, Graveyard, or Removed Zone. If this happens, the Monster Card's effects are not applied, and it cannot attack or change its battle position manually. Any maintainence costs must still be paid.
Declared cards on the field before "Prohibition" resolves are unaffected. Declared Monster Cards Set before "Prohibition" resolves can be Flip Summoned. Declared Spell or Trap Cards Set before "Prohibition" resolves can be activated.
After declaring a card for "Prohibition", the declared card cannot be changed.
If "Exodia the Forbidden One" is declared with "Prohibition", when a player has all five pieces of Exodia in their hand, they win the Duel. This is a win condition and cannot be prevented with "Prohibition".
If "Prohibition" declares "Sangan" or "Witch of the Black Forest", if the declared card is on the field when "Prohibition" resolves, its effect will activate when it is sent to the Graveyard. If the declared card is Special Summoned after "Prohibition" resolves, it "cannot be used", so its effect will not activate when it is sent to the Graveyard.
If "Nimble Momonga" or "Giant Germ" is on the field when declared by "Prohibition", when it is destroyed by battle and sent to the Graveyard its effect activates, but the Special Summoned "Nimble Momonga" or "Giant Germ" "cannot be used", so their effects will not be applied when they are destroyed by battle and sent to the Graveyard.
A Level 4 or lower Monster Card declared by "Prohibition" can be Special Summoned by "Cyber Jar", but it "cannot be used" so its effects are not applied and it cannot attack or change its battle position manually.
If "Jinzo" is declared by "Prohibition", the effect of a "Jinzo" that is already on the field is still applied. If a "Jinzo" is Special Summoned, it "cannot be used", so the effects of Trap Cards are not negated and Trap Cards can be activated.
If the effect of "Prohibition" is being negated, the declared card can be "used". If the declared card is on the field when the effect of "Prohibition" becomes active again the declared card on the field is unaffected.
↑Konami Judge Program Rulings are considered unverified, as their source(s) are not publicly viewable. They are not confirmed as applicable to the TCG until they have been officially published by Konami.