Bukkit

First you must open your HyperConomy config.yml and set the following configuration option:
unlimited-stock-for-static-items: false

Now any item that you assign static pricing will have infinite stock.

The following command will set the item "dirt" to static pricing:
/hcset static dirt

Note that the item still has stock in the economy, however the stock is never depleted.
A player may not purchase more than the economy stock value, in a single transaction.
So, if you want a player to be able to "/buy dirt 100" then you must make sure there is at least 100 dirt in the economy.
For example, to set 1000 dirt in the database stock:
/hcset stock dirt 1000

To set all items in your economy to static pricing use this command:
/hcset static all:true

To set all items in your economy back to dynamic pricing, use this command:
/hcset static all:false