I think there must be some other problem. If the old jar still exists, can you rename it so that it wouldn't be found? Then create the new one. I suspect that there is a problem creating the new one or the new one is being written to some other folder. If you create a new jar and issure the java -jar <jarfile> command in the correct folder, java won't go find the old version.
Are you talking about the jar file cache kept by the Java Plugin, in other words, applets or JWS applications? If so, check out the Java Plugin control panel; it has several settings for jar file caching, amongst them the option to turn it off completely.