How to adjust or obtain different IR Transmitter's distances?
You can limit the distance by lowering the LED current (or lowering its ON time compared to the OFF time), or by 'mismatching' the signal with what the receiver expects (change the frequency, saturate the receiver by continouus transmission, arrange for interferring signals), but you can't use that to set a reliably reproducible distance limit.

Think of it a speaking soft. How soft do you need to speak to make people hear you at 10 ft but not at 20 ft?