Clearing the cache will only temporarily help the space problem as it will rebuild itself.
Assuming you are on the current version:
- Delete inactive Members (I start with the 0 post members first)
- Prune some topics old topics, if there are ones you wish to keep, create an archive forum. Pruned topics are gone forever.
If you don't want to do either of those, consider getting a higher disk quota or move to a host with more space.
