How to prevent ppl from buying...

Wouldn't that be the opposite? Your scenario is having max mesos already with an item at 1 meso, while the question seems to be about how much mesos you need to have in order for a max meso item not to sell/in order to sell an item for max mesos.

I'm not entirely sure on this, but I would imagine you'd need more than the tax amount (3% of max mesos) in order for it not to sell. No experience on this whatsoever, so don't take my word on it, but that's just what makes sense to me.

If you set up an item for max mesos and another item for 1 meso, and the 1 meso item sells, no one can purchase the max mesos item.

/been there done that.

Edit: Personal recommendation; if you can spare it, sell something worth a couple mil for a mil to ensure no mistakes. My friend goofed on the pricing of his 21% dex clean cape and ended up selling it for 300 less than max, which would have been avoided had he put up an item for 1k or 10k or w/e instead of just 1 meso.