How to decrease nrf24l01 transceiver range?
No, not a fixed distance, but you can certainly reduced the expectable range, simply by reducing the output power of the transmitter, or by decreasing the sensitivity of the receiver.

These systems have no way of knowing distance – they can just "hear" or "hear not" the transmitter. How "loud" a transmitter is at the receiver depends on how loud it transmitted, or what happened to the signal from the transmitter to the receiver.

So, you need to tone down what the receiver can sense.

This can be either done by configuring the involved chips to use less power to transmit, or by using worse antennas, for example.

You'll find out that it's extremely difficult to achieve a reliable range by adjusting power – effects of random orientation, obstacles (e.g. humans) between transmitter and receiver and so on are much much larger than a few meters of difference in distance.

So, you might find out that it's conceptually impossible to define an "alert" radius without accepting a high number of false alarms or being too late to alarm.