You would need an RFID reader with a sufficient reading distance -- go for 10 inches to make it robust. The cheak 125 kHz ones will not do it. Maybe the 13 MHz ones will.
The RFID readers typically output serial data on a pin. You can hook that serial pin to the serial input of a microcontroller, such as the Arduino Leonardo. Write code for that microcontroller to recognize data coming in on the serial port.
The code would just activate an output pin, wait for an amount of time, and then deactivate the pin.
The pin, in turn, would drive some kind of controller that knows how to activate the scaring-device. You can't drive it directly from the pin, because digital pins on microcontrollers cannot deliver strong enough current.
So, approximately, you'd need something like:
1) a 12V 2A DC power supply of some sort (wall wart)
2) an Arduino Leonardo. You don't want the Uno, because its serial port is already used for programming it, so it would "fight" with the RFID reader
3) a RFID reader with suitable range, and a tag for it
4) an H-bridge or relay to drive the solenoid. Something like a Pololu 8835 motor driver would likely be sufficient
5) an actual solenoid (or air pump, or whatever) with current draw and voltage to match the driver. Or select this part first, and select a driver to match.
The Arduino is very easy to program. There are lots of tutorials on the http://arduino.cc/
website. You'd want to read the RFID serial input, and use the digitalWrite() function to turn on the output, call delay() to delay for a little while, and then digitalWrite() again to turn off the output.