You need to find the textures for the each of the particle effects - a separate one for each shield - and replace them with blank texture files. I know that all the relevant files begin with 'vfx', but I can't remember exactly which ones are used in the shields, and I don't have access to my home computer to check. You can see which textures are used by extracting the shield meshes from Morrowind.bsa and examining them in NifSkope.
Thanks, though it seems my problem has evolved a bit since my last post.
I did find a mod that seemed to do what I wanted in http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=3466. However, after installing the mod I noticed that when I loaded up nothing had changed. Given that this was a fairly old mod, my immediate thought was that I had already had a mod installed which was newer and prevented it from working. After going through my mod list and looking into suspicious mods I found the culprit: http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=6621
I hadn't thought of it prior, as I rarely play mage classes and so rarely used the shield spells prior. The mod is really great for making the things spell casters shot at me look nicer though, so I always liked it, and for that reason I would really like to keep it.
So I tried making a new mod that would simply overwrite the old one. This didn't work as Spec. Sorc. creates new statics to attach the effects to and uses those instead. So I tried editing the spec sorc file directly to remove the attached nif files, and I'm sure you can already guess what my inexperienced self caused to happen.
Anyway, despite the problem evolving I think your post is still the right answer for me. I will need a blank texture file to replace the old ones with, though always having been more of a balance/content user of the construction set I'm not sure what will qualify as a 'blank' texture. Either way, I'm off to download nifskope..
Thanks for both of your answers!