So I uninstalled Expanded Towns and Cities as it had floating Lantern Poles in Solitude.

Anyway, after uninstalling it, Ive noticed that these are still here (along with the triangular flags strewn across the main square.)

How do I uninstall this mod for good?

EDIT: I've already started 2 new games since to try and rectify the problem, but it hasnt worked.

Did you just uncheck the plugin, or did you remove all it's files? If you just uncheck the esp, a mod may still load.

Missing textures are the triangles.

Meshes, actually. Objects missing texture show up as solid purple.

Oops XD, I always get those two mixed up. Ok the meshes are missing meaning you deleted the meshes, but something is still "spawning" them, so therefore:

if that mod uses scripts, your game got baked with it.

Edit: But lets say you still want this mod but don't like the lights, which seems to the case. You can disable those in the esp, or to be on the safe side, take them out of the cells they are in.