I have tried both s3cmd:
$ s3cmd -r -f -v del s3://my-versioned-bucket/
And the AWS CLI:
$ aws s3 rm s3://my-v
Using boto3 it's even easier than with the proposed boto solution to delete all object versions in an S3 bucket:
#!/usr/bin/env python
import boto3
s3 = boto3.resource('s3')
bucket = s3.Bucket('your-bucket-name')
bucket.object_versions.all().delete()
Works fine also for very large amounts of object versions, although it might take some time in that case.