Here is what I can tell you.
You can delete anything that you want but your account may no longer work.
Nearly everything in
/username/home/ should be kept. I wouldn't delete anything here since you didn't personally put anything there in most cases. Nothing gets installed there.
You can delete everything in
/username/home/public_ftp/. Nothing is there by default and nothing is installed there either.
Now before you do anything else, uninstall any scripts that were installed using fantastico. Be sure to uninstall these scripts with the fantastico remove feature or you'll have to manually remove the entries from the install log before you can install another script with fantastico in that folder/subdomain/root. Really, you don't want to have to manually edit that file.
Once you have everything uninstalled CORRECTLY, then you can remove ALL subdomains. Don't just delete the folders. Use cPanel to remove the subdomains. If you just delete the folder, then the server wont know that you no longer have the subdomain.
Again in cPanel, remove any or ALL email accounts that you no longer need.
Remove as many advanced website settings you have selected like hotlink protection, forwarding, password protected folders, etc. (These will basically be gone when you delete any .htaccess files but removing them first will leave fewer chances of missing something.)
Delete your databases and database users. Even though fantastico removes scripts, it usually forgets to delete the database that it added. Databases use up a lot of space especially if it isn't needed.
Now you can safely delete everything inside of
/username/home/public_html/! You can even delete
cgi-bin/ if you want!

to save yourself the time needed to select everything, you could just delete
public_html/. I know everyone else will tell you not to.

It isn't that big of a deal, just replace it later. I'll explain in a second. You should probably save a copy of .ftpquota and error_log to your local system.
Well, there you go, everything is all gone. Back to square one.
Now if you did happen to take my advise and you did in fact delete public_html then I really sorry to tell you that I was just kidding!

No, just create a new folder and name it public_html. Then open that up and create a folder and name it cgi-bin. The you can copy .ftpquota and error_log back to your public_html folder.
Okay, you're done with that. It will take between 24 and 48 hours but your disk usage should show around 25MB or less. I don't remember how much space an "empty" account uses.
Feel free to repopulate your account now.

Hope This Helps!
vujsa