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

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",
        }
}

You should empty the bucket:

$ aws s3 rm s3://bucket-name --recursive

Then delete the Bucket

$ aws cloudformation delete-stack --stack-name mys3stack