How do I remove Dynamic pricing from these custom vending machines?

Currently you can only remove it globally via convars.

> find npcvendingmachine
Variables:
 npcvendingmachine.dynamicpricingenabled Whether to run the the dynamic pricing system (True)
 npcvendingmachine.intervalhours How many in game hours are checked when looking for price increases. Max 72 (3 days) (6)
 npcvendingmachine.maximumpricemultiplier The maximum point that a price can increase to (2 = 200%) (2)
 npcvendingmachine.minimumpricemultiplier The Minimum point that the price can drop to (0.5 = 50% off) (0.5)
 npcvendingmachine.pricedecreaseamount How much to decrease the price for if it is underselling (0.05 = 5%) (0.05)
 npcvendingmachine.priceincreaseamount How much to increase the price by if it is selling a lot (0.05 = 5%) (0.1)
 npcvendingmachine.startingpricemultiplier What discount surcharge should be applied to items when the server starts (2)

Commands:
 npcvendingmachine.printallpricechanges(  ) Print out all current price changes on the server
 npcvendingmachine.resetdynamicpricing(  ) Resets the state of all discounts and surcharges from NPC vending machines

If you want to do it for select vending machines, that could be in scope for Custom Vending Setup since most likely you are using that to customize vending machines spawned by MA.