I uninstalled clamav with
apt-get remove --purge clamav
but still I have this 100MB folder:
du -shc /var/lib/clamav
Why is this not deleted on purge? And how can I find out if some other installed program still uses this folder?
clamav on at least Debian (you don't mention what distro you're using) doesn't contain the database. For that
clamav has a dependency on
clamav-freshclam | clamav-data so make sure that both of those are also purged.