What happens exactly when you flag an item with "dont create"?

because i was searching for a way to make some items still be in the files but not appear in the game

Tagging a Unit file with "DONTCREATE:true" simply excludes the Unit from most spawn events that operate on a UnitType basis.  It is mostly used to indicate Unit files whose only purpose is to define inherited values for other Unit files (`BASEFILES`, in other words).

The flag does not work correctly on Merchants; many items tagged with "DONTCREATE" will still appear in Merchant inventories. 

The only way to guarantee an item does not appear in-game is to make it invisible to the game engine itself.  More on that below. 

Good news : Thanks to @Chthon, i managed to find a way to stop spells from spawning, in the mod panel go to remove files and then exclude the spells entirely. This will probably remove any existing copies from a save but this seems to be the only way to stop them from spawning since spells are massively bugged and setting them to rarirty 0 and DONTCREATE still causes them to spawn.

For easier reading, open up:
GUTS –> Right Panel –> "Mod" Tab –> "Remove Files" Sub-Tab 

Do not be afraid of the labeling; all this sub-tab actually does is add a field to the MOD.DAT file in your mod, listing files the game should ignore.  The files themselves will still be present in the game data—but the game itself will act as though they are not. 

You can remove vanilla files this way, too—so keep in mind you will have to scroll through every file in the game (including vanilla files), to find the files you want ignored.