If it says spell not found on x, then I'm pretty sure you did the command correctly but just have the wrong spell ID. Most of those generic spells have many different versions (and different IDs) depending on the damage level. You can see this in action if you use the psb command to totally fill in your own spell book (save first!), and then go look at how many flames spells (for example) there are... maybe 10 or 12 of them in total, iirc. A couple of them are trap spells, usually only one is meant for the player, and all the rest of that type are the ones that NPCs use. To get the right one that your NPC is using you would probably have to load up the creation kit, go to the spells section, and copy down all the IDs for all the different level versions of a given spell, then load up skyrim and try removespell with each one until you get a hit. Otherwise.. I don't know. I don't think there's an easy way to tell what version of a spell a certain NPC is using, and generic NON-player spell IDs aren't usually in high demand on the internet wikis/cheat sites.
You may also get lucky by viewing your named NPC in the creation kit, then looking at their spells tab to see their default spells (the IDs of those will be included). As long as your NPC hasn't leveled up then the default spell IDs shown in their spells tab should work. If they have leveled then you can bet they've moved on from those base spell IDs and onto one with more damage.