The repeated message was probably logged into /var/log/kern.log* and
/var/log/syslog*. You can see the sizes of these files with the following
command (you can use a graphical file manager too):
$ ls -lSh /var/log/kern.log* /var/log/syslog*
If they are indeed large and you do not think you will ever look at them
(your past problems are past), remove them (otherwise, copy them somewhere
else and then remove them):
$ sudo rm /var/log/kern.log* /var/log/syslog*
Other classical ways to save disk space on the root partition (i.e., the
partition mounted at /):
Remove packages that are not needed anymore (dependencies of packages you
removed): $ sudo apt-get autoremove
Remove .deb packages (only drawback: you need to download the .deb package
again to reinstall it): $ sudo apt-get clean
Remove packages relating to older versions of the kernel (new versions of the
kernel do not substitute older versions, unlike any other package) such as
"linux-image-*", "linux-headers-*", ... with a version number that is not the
largest (in the dictionary order: the leftmost dot-separated numbers are the
most significant) or the second largest (for safety: if the latest kernel
does not boot or is defective, you can boot the older one that raises no
issue).
