[r] remove kernel on jupyter notebook

Run jupyter kernelspec list to get the paths of all your kernels.
Then simply uninstall your unwanted-kernel

jupyter kernelspec uninstall unwanted-kernel

Old answer
Delete the folder corresponding to the kernel you want to remove.

The docs has a list of the common paths for kernels to be stored in: http://jupyter-client.readthedocs.io/en/latest/kernels.html#kernelspecs