I would run the following:
iptables -t raw -D PREROUTING -i l2tp0 -j MARK --set-xmark 0x00000000/0x7f800000
Iptables -D deletes the equivalent entry added with the -I in the previous command. The "-t raw" part specifies the table to use (raw, in this case)
you can also use "iptables-save > [some-file-name]" and then "iptables-restore <[some-file-name]" to save / restore the current iptables state.
Just make sure the [some-file-name] is saved somewhere that would survive a reboot if you want.
you can always use a "diff" between the old file and the new one to see the differences