completely uninstall r linux

You might want to check on all currently installed R packages.

You can list all packages whose name starts with "r-" like this:

dpkg -l | grep ^ii | awk '$2 ~ /^r-/ { print $2 }'

To uninstall all of them, pipe the output to xargs apt-get remove:

dpkg -l | grep ^ii | awk '$2 ~ /^r-/ { print $2 }' | xargs apt-get remove --purge

The R binary (well, front-end script) is part of the r-base-core package which contains the core R system.

The package r-base is a so-called virtual package which exists to just pulls other packages in. Removing it does not remove any parts of the R system --- for which you need to remove r-base-core.