as a free and unannoying dyndns provider, like nsupdate.info, you have one problem:
you never know whether a dynamic host entry is still used in case you do not receive updates for a while.
it could be unused, but also it could be a cable internet user whose ip rarely or never changes (or it could be even a static ip and the user just using your service to get an easy hostname although it's not really "dynamic").
so, basically, cleanup of your host database is impossible, because you do not know what's still alive and what not.
on the client side (IPFire side) you could help a bit by doing unconditional updates in long intervals.
like e.g. if your (well behaved) update client did not send an update since 1 week (because your IP did not change), send one! This will result in a nochg response, but as long as you do that in long intervals only, this is no problem.
IPFire offers this feature by using cron and performing a forced update once a week.