Removing the cuda list from the apt sources should do the trick.
So, the following in the most generic case:
rm /etc/apt/sources.list.d/cuda.list && apt-key del 7fa2af80
Note that the exact list name is dependent on the exact repo you added, for example for me it was:
rm /etc/apt/sources.list.d/cuda-ubuntu2004-x86_64.list && apt-key del 7fa2af80