Hey Arnold.
I think there are actually laws in several countries that prevent people from selling for example a car that would be named "car". My understanding is that using a common word describing exactly the same thing as the product, for a brand name, is most likely illegal in many countries.
You can't sell a piece of software and call it "SOFTWARE", but you can call it "Windows".
I mean, from a trademark point of view.
You could have named your Shell "car", but naming it "SHELL" was probably not such a great idea.
As a conclusion, I don't think you can ever trademark the name "SHELL" or claim any kind of property on that name associated to a shell type of software. Please see what happens when Apple tries to claim ownership for the word "appStore" : they lose lots of money in court and their competitors mock them.
That being said, you can probably try to contact the guys and ask them to lowercase their text, but in all honesty, you shouldn't be offended by that.
Personal opinion about your threats of discontinuing for such a trivial matter: please don't be that kind of guy