Whenever a tyranid or traitor stands still and reverberates, you'll notice a circular icon with a skull inside appear. If it fills from bottom to top, then more threats will appear.
Before that fills, you must lay as much hate as fast as possible. If it's enough to stop them without killing them, then that'll stop the reinforcements, but you should kill the enemy soon after just to be sure it'll count.
Charged plasma headshots, grenade spamming and other high yield attacks usually help me the best for preventing reinforcements.
Good hunting and godspeed