I do think that 500 post will be a suitable amount of post for a free VPS which does not even need to pay a penny.
With this high amount of post count requirement, people will actually prefer to go for cheap VPS.
And when they pay for it with their own money, they will be more careful on what they going to host, as they will fear to losing the VPS which they pay with their own money.
With this high amount, those email spammers will not going to attempt to get one for setting up a spam server, as they will think it is wasting a lot of their time. They are intended to use the VPS for a few days for spamming out emails anyway, so it is going to be not worth for them to post that much forum post to change for a VPS which they going to use only a few days.