Depends what is on them really. If its just grime you cannot beat a good scrub with warm water and soap. If its a bit of cement then a mild solution of pation cleaner will do the job. If its heavy bitumen then it needs to be wormed up and scraped off then cleaned with white spirits. See our project section www.diydoctor.org.uk/projects.htm and look for cleaning paving and patios and removing cork tiles as the process for cleaning the adhesive is the same.