Where possible, change light bulb colours.
In offices that have lamps, change standard white light bulbs to a rainbow of colours. The trick in this prank is not the new colour, but the feeling of intrusion the staff member will feel for their particular lamp. Desk space is territory in an office environment, and many employees are more than happy to defend their territory.
For best results, discard any stores of standard light bulbs that may be used as replacements. This will force staff members to either adapt to the colour or deal with standard illumination if available. This prank has specific applications in environments with digital image production. They often rely on specific illumination to best view their work.
Research the effect of colour on people if you wish to personalize the experience. If you have a particular target in mind, change all lamps to blue, and your targets to red. The double change (colour of bulb, and becoming outside the group) will turn up the heat on your target. They may begin to exhibit symptoms of a persecution complex that will lead them into hanging themselves in the office.
If you are not invested enough in purchasing a variety of light bulbs, simply remove all of the light bulbs.