Can I force CloudFormation to delete non-empty S3 Bucket?

I think your DependsOn is in wrong resource, at least it did not work for me properly because on stack deletion (via console), it would try to force bucket deletion first which will fail and then will attempt to delete custom resource, which triggers the lambda to empty the bucket. This will empty the bucket but the stack deletion will fail because it attempted to delete the bucket before it was empty. We want to initiate custom resource deletion first and then attempt to delete bucket after custom resource is deleted, so I did it like this and it works for me:

myBucketResource:
  Type: AWS::S3::Bucket
  Properties:
    BucketName: my-test-bucket-cleaning-on-delete

cleanupBucketOnDelete:
  Type: Custom::cleanupbucket
  Properties:
    ServiceToken: arn:aws:lambda:eu-west-1:123456789012:function:clean-bucket-lambda
    BucketName: my-test-bucket-cleaning-on-delete
  DependsOn: myBucketResource

This way you ensure the bucket deletion does not come first because there is another resource that depends on it, hence the depending resource is deleted first (which triggeres the lambda to empty the bucket) and then bucket is deleted. Hope someone finds it helpful.

You can create a lambda function to clean up your bucket and invoke your lambda from your CloudFormation stack using a CustomResource.

Below a lambda example cleaning up your bucket:

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import json
import boto3
from botocore.vendored import requests


def lambda_handler(event, context):
    try:
        bucket = event['ResourceProperties']['BucketName']

        if event['RequestType'] == 'Delete':
            s3 = boto3.resource('s3')
            bucket = s3.Bucket(bucket)
            for obj in bucket.objects.filter():
                s3.Object(bucket.name, obj.key).delete()

        sendResponseCfn(event, context, "SUCCESS")
    except Exception as e:
        print(e)
        sendResponseCfn(event, context, "FAILED")


def sendResponseCfn(event, context, responseStatus):
    response_body = {'Status': responseStatus,
                     'Reason': 'Log stream name: ' + context.log_stream_name,
                     'PhysicalResourceId': context.log_stream_name,
                     'StackId': event['StackId'],
                     'RequestId': event['RequestId'],
                     'LogicalResourceId': event['LogicalResourceId'],
                     'Data': json.loads("{}")}

    requests.put(event['ResponseURL'], data=json.dumps(response_body))

After you create the lambda above, just put the CustomResource in your CloudFormation stack:

 ---
 AWSTemplateFormatVersion: '2010-09-09'

 Resources:

   myBucketResource:
     Type: AWS::S3::Bucket
     Properties:
       BucketName: my-test-bucket-cleaning-on-delete

   cleanupBucketOnDelete:
     Type: Custom::cleanupbucket
     Properties:
       ServiceToken: arn:aws:lambda:eu-west-1:123456789012:function:clean-bucket-lambda
       BucketName: !Ref myBucketResource

Remember to attach a role to your lambda that has permission to remove objects from your bucket.

Furthermore keep in mind that you can create a lambda function that accepts CLI command line using the lambda function cli2cloudformation. You can download and install from here. Using that you just need to create a CustomResource like bellow:

"removeBucket": {
        "Type": "Custom::cli2cloudformation",
        "Properties": {
          "ServiceToken": "arn:aws:lambda:eu-west-1:123456789000:function:custom-lambda-name",
          "CliCommandDelete": "aws s3 rb s3://bucket-name --force",
        }
}