How to disable sprint?

You can do something like:

Event Trigger - (on player update tick)

while

Get Item from Armor Slot 0 of Event/Target Entity = (custom armor boots)
and
Get Item from Armor Slot 1 of Event/Target Entity = (custom armor leggings)
and
Get Item from Armor Slot 2 of Event/Target Entity = (custom armor chestplate)
and
Get Item from Armor Slot 3 of Event/Target Entity = (custom armor helmet)

do

Set sprinting of Event/Target Entity to false

For every tick the player is wearing a full set of your custom armor, the script will set the sprint state to false. IF the player tries to sprint, it will immediately set the sprint state to false. I have not tested the script, so I do not know of any errors currently.

You could give them slowness I. This actually won't slow the player's walking speed down, but it will stop them from sprinting.