Just speculating here, but is it possible that the texture might have been applied locally? So perhaps you have deleted the global texture but not the local one....
Also, if you applied the texture on a group, deleting the group should do the trick.
Finally, you should select the object, click on Render > Shader, then see if anything is connected into the inputs of the various parameters. If so, right-click on them and choose Disconnect.
Hope this helps
Salutations - Cheers
Bernard Lebel