This is covered in the FAA's AIM, Chapter 5, Section 6. The UK has similar procedures.
First, the standard guard frequency is 121.5 or 243.0 MHz. If you aren't listening to this frequency and get intercepted, you should start listening.
Methods like LED signs or possibly as whiteboard as you suggested can also be used. Otherwise, they maneuver their aircraft to send standard signals. See the guides above for details on the signals sent by interceptors and how they should be acknowledged. Basically:
- You have been intercepted, follow me
- Interceptor: Rocks wings (and flashes lights irregularly at night)
- Intercepted: Same
- You may proceed
- Interceptor: Breakaway, climbing turn away from aircraft
- Intercepted: Rocking wings
- Land here
- Interceptor: Circling airport, lowering landing gear, overflying runway (landing lights on at night)
- Intercepted: Lower gear, land
The intercepted aircraft can also send signals. The interceptor will respond with one of the signals above.
- Designated airport is inadequate
- Intercepted: Raise gear, flash landing lights
- Cannot comply
- Intercepted: Switching on and off all lights, but distinct from flashing lights
- In distress
- Intercepted: Irregular flashing of all lights
In the case of the Washington, DC Special Flight Rules Area (DC SFRA), there is a Visual Warning System (VWS) of red and green lights that will let the pilot know that they shouldn't be there. The pilot is supposed to turn directly away from the center of the SFRA. If they are in contact with ATC they must advise that they have been illuminated by the system, otherwise they should contact ATC on a guard frequency and identify themselves.
Of course, an interceptor can't force an intercepted aircraft follow instructions. It goes on to explain what will happen if you don't follow those rules, which probably apply in any situation where the risk is too high:
Further noncompliance with interceptor aircraft or ATC may result in the use of force.