Open up the image in photoshop.
Mode > Adjustments > Levels
Select the white point eye dropper (the one selected in the attached levels.jpg
Click on the white background
This sets the maximum level of white at that point so clicking on a darker area will high key out more of the colour, play with clicking at different points in your image if you have a non-uniformly lit background.
I've included before and after jpgs to show the effect of setting the proper whitepoint.