To have expired API tokens automatically deleted, install the following cronjob.
To do this, call as root:
# crontab -e
and insert the following lines:
1 * * * * php -f /path/to/vimp/symfony --framework:clean-apitoken
>> /path/to/vimp/log/clean-apitoken
.log 2>&1
Where /path/to/vimp should be replaced with the appropriate path to the VIMP directory on your server.