When you have some OpenVPN Roadwarriors configured in your IPFire, some connection data will be stored through the collectd daemon. For each connection the traffic data is stored in RRD files which are represented in nice graphs for each OpenVPN Client.
You are able to see, when a client connected and how much traffic was generated. The data is selectable for Hour, Day, Week, Month and Year.
These can be removed when obsolete by the following steps (a knowledge of Linux is required) :
1) enable ssh access
2) log into your ipfire machine
3) su to root if you haven't logged in a such.
4) run /etc/init.d/collectd stop
5) cd /var/log/rrd/collectd/localhost to where the files are
6) rm -r the appropriately named directory(ies) (be sure you are certain you have have not made an error in the command...it can be very destructive)
6.1) cd in to the appropriately named directory
6.2) rm the files there one at a time
6.3) cd ..
6.4) rmdir the appropriately named directory
6.5) run /etc/init.d/collectd start
6.5) check the result is as expected.
6.6) run /etc/init.d/collectd stop
6.6) repeat as needed.
7) run /etc/init.d/collectd start
NOTE: this is a not a perfect solution, but does work. In my experience.