It is always preferable to be friendly and flexible.
Before you jump to make this a formal go by the rule type of response, first understand what is being expected, and why you would'nt want to do it. If it is a task that would not add value to you or inappropriate for your skill and experience, try to handle with apt.
One way would be to perform that task once very well, get it acknowledged and the next time politely inform that you would want to focus on responsibilities at hand and try to find out why this is being assigned to you regularly, in your opinion who should be doing it, and why it is not being assigned to them.
If the assignment is making you feel hurt, try to understand what is going on.
Management usually assigns work to people who they think are confident of delivering. If they are assigning you irrelevant work, or work that you consider low profile, may be you are not being friendly and favoured. Try to work on that. Assertions quoting rules, roles and responsibilities may not lead you to favourable response.