Definitely the best way to uninstall all pypi packages in a conda environment is:

conda activate <your-env>
conda list | awk '/pypi/ {print $1}' | xargs pip uninstall -y

From the docs:

To uninstall Anaconda open a terminal window and remove the entire anaconda install directory: rm -rf ~/anaconda. You may also edit ~/.bash_profile and remove the anaconda directory from your PATH environment variable, and remove the hidden .condarc file and .conda and .continuum directories which may have been created in the home directory with rm -rf ~/.condarc ~/.conda ~/.continuum.

Further notes:

• Python3 installs may use a ~/anaconda3 dir instead of ~/anaconda.
• You might also have a ~/.anaconda hidden directory that may be removed.
• Depending on how you installed, it is possible that the PATH is modified in one of your runcom files, and not in your shell profile. So, for example if you are using bash, be sure to check your ~/.bashrc if you don't find the PATH modified in ~/.bash_profile.

You can use conda remove --force.

The documentation says:

--force               Forces removal of a package without removing packages
                      that depend on it. Using this option will usually
                      leave your environment in a broken and inconsistent
                      state