Toribash
Since you didn't really clarify exactly what you're having trouble with I'll answer everything I think you could have meant.

1.) If you mean an in-game command like "/opt (here)" thing there isn't one for trails currently.

2.) If you meant how do you de-activate the item you can select the trails and click "Deactivate" here.

3.) If you're talking about de-activating textured trails you need to upload a single black canvas to the textured item in order to deactivate it.
I included a black canvas you can upload in this post. ;)
Attached Images
Blank_Texture.jpg (12.1 KB, 5 views)
[SIGPIC][/SIGPIC]
<@Opeth> No, they should get the worst texturer
* @Opeth suggests <censored>
<@hanz0> good suggestion
<@Opeth> I don't understand how you can make textures for so long and not improve in the slightest
<@hanz0> it is because his learning curve is more of a learning horizontal line
Hmm, i too have been wondering this. I think he means a way to disable coloured trails too, for screenshots. The black canvas could work for that with some mucking around though. I guess you could do item.dat editing .___-.
Opeth: black and white/greyscale textures should upload correctly now, so we don';t have a way to break textures now :p


[T]
#hi


ohnoes.

Seriously?
I thought that just applied to textures that were black & white?
So, in theory pure black should still work.... right? D:
[SIGPIC][/SIGPIC]
<@Opeth> No, they should get the worst texturer
* @Opeth suggests <censored>
<@hanz0> good suggestion
<@Opeth> I don't understand how you can make textures for so long and not improve in the slightest
<@hanz0> it is because his learning curve is more of a learning horizontal line