Jeanne,
by BlueHost account getting full, I assume you're referring to the number of files? as they dont actually limit the file size, but there is limitation on number of files that one can keep on shared servers, which last time I know was 50,000 (but they safely allow up to 100,000)
Of course you can delete CACHE, LOCALCACHE and as well SESSION folder, as that will have tiny txt files sitting there and doing absolutely nothing, but do take care to delete all this when no one is using moodle (or put in maintenance mode), as deleting session folder at least, you'll be logged off from moodle as well.
Deleting CACHE/LOCALCAHCE folder will take a few seconds for your site to load as it will re-build cache, but deleting SESSION folder will have no effect apart you getting logged out of your session, so do try deleting SESSION first, and see if the number of files have came down, if not then proceed with CACHE folders, but do remember, for BlueHost, it takes some 4 hours to refresh the file count.
You can always have a look inside (may be mentioned as file count in BH's panel), and see which folder/folders are holding most number of files as well.