Is there a way to avoid animations included in meshes from playing, from the esp or by a script function?

This completely depends on the object in question -> if the animation is a cycling anim then it will never turn off, if its a managered anim then there should be an off animation you can call.

or just unpack that .nif and alter it to how you want it to play and create a new object with it.

It's the first case, a cycling animation. Is there a good tutorial on how to export to blender, change and import again a mesh?

Depending on what your trying to do if all you want is to disable the anims you can open the .nif in NifSkope and set its Controllers from Cycle to Clamp then they will not turn on or just delete them entirely -> if your looking for a way to set them up as Managered so they can be turned on and off by script thats a whole different problem.

That would be perfect, I don't need the animation at all. But I can't open the mesh in NifSkope. Any suggestion for that? a workaround, a pluggin maybe? (maybe the version I'm using -1.022- is outdated)

http://sourceforge.net/projects/niftools/files/nifskope/1.1.0/nifskope-1.1.0-rc6-i686-w32.7z/download is the latest version if NifSkope it should include the latest Nif.xml though I do not know as I do not use the released versions of theirs as ThrottleKitty keeps renaming Fields form what they are in the game to some unknown to me why the f**k its getting changed for reason and I can not stand when things are not named what they are suppose to be named so I stopped using their nif.xml a long time ago...

Thanks