This is most likely not the best approach, but it is an approach.
You could create a trigger that covers the whole map, get a list of all the units within the area of the trigger (the whole map); then create a trigger that covers a 1500 m radius around the player and get a list of all the units within this area. Then, subtract the units near the player from the list of units on the map and and delete all the units in the resulting array.
Alternatively, if you're using BAS-f (which I highly recommend at least for single player missions) and have been maintaining the list of EAST units when you create new units (as instructed in the manual), then all you need to do is determine what units are close and subtract them from the total list to have a list of units to delete.