You could just remove the displaying of post counts. And instead of basing your ranks on postcounts, how about basing them on time in the community?
Someone who has been around for 2 years, but doesn't post a lot, should deserve more credit and respect then a newbie with an average postcount of 10/day, even though this newbie might have a 'higher' rank.
As a former forum admin myself I found that this really cuts back on spamming since people stop caring about raising their postcount.