The correct way of giving credits is listing all authors of things you did not create yourself AND list what objects they made. If you don't credit that way you are, more or less directly, stealing.
Example of crediting (from my map where it is a quest, color codes just replaced with bold tags):
Blade.dk: Map, terrain, spells, AI, Inventory System, some icons, some models, and everything not listed elsewhere.
Vexorian: The Caster System, The Hero Selection system, the SimError, GetPlayerNameColored, Bounty and GetRandomPlayerName functions, the Storm, Earth And Fire and Airy Charge icons.
Mc !: The Blaze, Arcane Pyramid, Select Hero, Nuclear Strike, Bombs, Place Bomb and Explode Bombs icons.
weaadder: Bonus Mod.
Daelin: The map preview and the Duel Challenge icon.
KaTTaNa: AddTimedLightning function.
Cookie: Hero Glow models.
Eusira: Manastrike model.
EdwardSwolenToe: Electrical Shield model.
gookywooky: Holy Vengeance model.
Elunes-Guardian: Spit Blood model.
ArcanisteR: Burning Charge icon.
KoTuK: Holy Vengeance icon.
People that uses other people's stuff without crediting are idiots.
And by the way, why not delete maps with stolen contents from the site? It would help against your space problems, make the section cleaner and force people to give credits if they wanted their maps on the site. I hate that the site condones stealing, seriously, idiots are using the stuff I've spend hours on without even adding a stupid little reference to me, and their maps are accepted anyway.