To delete all Helm releases in Linux(in Helm v2.X) with a single command, you can use some good old bash. Just pipe the output of helm ls --short to xargs, and run helm delete for each release returned.
helm ls --all --short | xargs -L1 helm delete
Adding --purge will delete the charts as well, as per @Yeasin Ar Rahman’s comment.
helm ls --all --short | xargs -L1 helm delete --purge
On Windows, you can delete all releases with this command, again, with --purge deleting the charts as well.
helm del $(helm ls --all --short) --purge
Update: As per @lucidyan comment, the --purge arg is not available in Helm v3.