Your office accountant Colleen is very active in a local councilman's campaign for reelection. You have seen her on several occasions at other employees' desks engaging them in political debate and trying to convince them to vote for her candidate.
While Colleen's activity may seem harmless enough (especially since she's not asking for contributions), she isn't doing her work and is interfering with the work of other employees. If Colleen were doing this kind of thing on her lunch time or break times, it would be okay, but during work time, you can require that Colleen not engage in political activities.
Continuing with our example, if Colleen persists in hounding people to vote for her candidate after they have requested that she stop, her actions could be construed as harassment. Take action against this behavior.