I would like to be able to place a ruler (and other things) against the board without those objects writing on the board, but I need to be able to write. I believe there is a way to set the board to accept input only from the pen so that if I accidentally touch it, or need to hold the ruler up against it, it doesn’t write on the board — but I have looked all the places I can think that it might be . . .
If the board in use is one of our ActivBoard Touch models, then any object placed upon the surface of the board (the stylus, a finger, a ruler, a tennis ball, etc.) will trip the IR sensors in its frame, and be registered by the computer as an input. In this case, try using the Ruler tool (Tools > Maths Tools > Ruler) if a straight edge is needed. There is no setting to adjust so that the computer to only accepts input from certain objects.
If any of our previous models of board are in use, then only input from the ActivPens or a touch input (on a 500 series board) should be the only way to operate the computer connected to that board. If the computer is responding to a ruler making contact with the surface of the any non-ActivBoard-Touch board, then we would need to troubleshoot this issue further as a possible issue with the hardware in use.