Can you explain a lot more about what the issue actually is? When you say you can't clear the cache, what does that mean? Does it just show up in the Cache Manager area that some extensions have cached data or is something on your site not updating properly?
If all else fails, disable caching in the Global Configuration area and set the cache timeout to '1'. That should disable cache, but for extensions that use their own caching, the 1 should make them only cache for a minute.
Hopefully that helps, but if not, please explain in more detail about what is happening on your site so we understand better.
Thanks,
Alex