Jburns has the best suggestion. Plotter doctor does great work.
If you need to clean those in the future try using a slightly damp cotton rag and then clean off any bits left behind.
If the roller is in really bad shape you can use a product called "Rubber Renue." Smells terrible but it pushes out contaminants by rehydrating the rubber, which you then scrub off using a slightly damp cotton rag. Don't plan on using the plotter for at least a day or two after rehydrating, needs to outgas.