There's a personal ward that blocks hostile magic, but it can be tricked into thinking a spell is non-hostile, success rate depends a lot on the relative skill of the casters - there are usually other ways to slip a spell through a ward like that, also generally dependent on the skills of the casters. Generally you have a bit better than even success rate blocking spells from someone at your own skill level. I've used it before, - they send the words and gestures -, If you remove the exceptions for friendly magic the success rate goes up a lot but it can interfere with other active spells. There's also antimagic fields that shut down all spellcasting in an area - suppress ongoing spells, too - usually not very large, not very long-lasting. I've seen them after they were cast but I have no idea what the spell itself looks like. They can be made more permanent.
Divinations like True Seeing will usually see through invisibility, but there are ways around that - I had a ring that did it before I came here - and while I don't know them now you should expect that if I turn evil I will probably research them.
That's it for general things I know of, but a lot of Spheres can counter themselves - Warp can block teleportation, for example. I think necromancy is one of those spheres, but conditional on necromancy turning me evil you probably shouldn't be very trusting of any of your researchers who claim to have figured out such a ward.