Dataset/Histories can be downloaded from servers as individual files or archives. These can then be permanently deleted on the public server to free up space in your account. It is recommended to keep backups of your important data anyway.
Downloaded data can always be reuploaded to the public server or your own local or cloud Galaxy server later as needed.
Some dataset types in your account can be converted to compressed formats (and the original perm deleted). Examples: fastq to fastq.gz, SAM to BAM, Wig/Bedgraph to bigWig, etc - but this can be tedious to do per-dataset and will not be free up as much space at once.
Thanks! Jen, Galaxy team