Here is a step-by-step tutorial:
1. Make sure that you can view hidden folders (in a folder press Tools, Folder Options, View, Hidden Files and Folders, Show hidden files and folders).
2. Install the Siemens Data Suite if you haven't already done so.
3. Click on My Computer, then Mobile.
4. Open the folder Cache on your S55.
5. Delete the file graphcach
6. Make a new folder called graphcach
7. Et viola! Done! You won't have the ever-growing cache file!