Is there any way to do so? there's an scv in my arena map that can spawn clones of itself with very low hp, and then some kid found out they can be repaired for free. any way I can prevent this without changing the unit?

To repair anything, it costs Minerals and/or gas.
Depending on the units cost-
If Resources are not part of the game, just change the unit cost to some minerals.
If only minerals are part of the game, just change the unit's cost to some gas.
I think that theres a way to disable an SCV's Ability to Repair... but i have no idea how.

If you spawn the SCV's by trigger, set their build cost to 9999 minerals and 9999 gas and Max build time. They will repair SUPER slow, FOR TONS OF MONEY! And the gas is there incase you got a mineral hacker.

ah, thanks a bunch guys, I forgot that build cost aslo = repair cost