Is there a way (in the GUI) to manually delete DHCP lease? The DHCP server seems to "remember" even the expired leases, but I would prefer if they were removed, or there was a way to delete them.
From my experience using other DHCP servers this is a GOOD thing.
pfSense remembers the leases for much longer than the expiration time, this means that if you remove a computer from your LAN and leave it offline for a long time, and then reconnect it... it will probably receive the same IP address. If pfSense needs those leases it will reassign them as needed, but if they are not needed it will leave them.
This will prevent IP address conflicts and will also allow your port forwarding to work on computers that are not always on the LAN.
The impact of this is a few KB of disk space used (insignificant) and almost no RAM or CPU overhead. If you want it cleaned up for asthetic purposes then I would say you are circumventing legitimate functionality.
I have my complaints about the DHCP implementation on pfSense, but this is not one of them.