You may have to actually add a weapon visual to get rid of the flames.
I believe that the regular weapon VFXs are tied (via a 2da) to the bonus damage property. So, if your weapon has a damage bonus of a certain type of at least a certain amount (say 1d6 or higher), the weapon will have the VFX, unless you specifically add a different VFX by means of another...
ItemPropertyVisualEffect(ITEM_VISUAL_*);
If you are using one or more haks for your module, you can edit iprp_visualfx.2da to add a new "none" VFX:
Label Name
0 Acid 16877231
1 Cold 16877257
2 Electrical 16877248
3 Fire 16877256
4 Sonic 16877255
5 Holy 16877237
6 Evil 16877239
7 NoneNull 16877238
* Ignore my .tlk file references for the names...those are from my own version of the file that I happened to have open. For the one for "none", I'm sure there's a dialog.tlk line with that word.
Now, for the script, instead of using the ITEM_VISUAL_* constant (which matches up with the 2da line numbers), you can just use "7".
The standard VFX will still show up in the toolset, but in-game it'll be overridden by the "new" VFX...which doesn't exist (and never will unless you are running the game with NWNCX and have the new visuals added in).