You can tell Cache Cleaner to empty that folder. It's a ready-to-use option in the system plugin settings.

But the real question here is why that folder is filling up!

Normally that folder is used to story files/folders that are only used during a pageload, and get deleted at the end of the page load, when the process is done.
If that is not happening, there might be an issue with the read/write permissions of the tmp folder on your server.
Ask your host about it.