Viet was so angry that he couldn’t restrain himself from punching Koichi and screaming, "Cut it out!"
You know how いい加減 is unfounded or pointless? Because if you add and then decrease you haven't really done anything? Well, this is like saying "do the pointless!" Okay, that doesn't make any sense. This is a phrase that means something similar to cut it out or cut the BS!
The reading is the same as いい加減, which you (hopefully) learned earlier.