I have an '06 (I think) Marz MZ-1 and I have the same issue with softness. It seems that the elastomers get soft after a while. Actually it is a really nice and soft fork, buttery smooth. The bad thing is that there is no compression damping - I hit the brakes and the fork dives all the way down giving me that funny feeling of about to go over the bars. The MZ-1 has a lockout, which works very nicely (it even works to lower the travel if you compress the fork and then lock it out), but unfortunately it is either fully locked or not - no in between.

AFAIK there is not oil adjustment you can do (so says the manual). The rebound cartridge is sealed and not serviceable. If you get a hold of stiffer springs, that might do it, but if I were you I would use this an opportunity to upgrade forks (try a Rock Shox Tora 302 or 318).

Perhaps the fork would be OK for lighter riders under 150 lbs?