You're not freeing space by deleting files if they continue to exist in snapshots, you can either; 

- wait for snapshots to get rotated out and destroyed

- destroy unused snapshots, use `zfs send` to backup to external media first?

- mount each snapshot, then remove the offending files from each; which seems tedious, and prone to mistakes.

  c.f. `zfs diff` to find differences between snapshots

Cheers, Joel