How can I free Spice capacity?

Let's have a look at the datasets and ingestions first. You can check the amount of SPICE capacity used by a particular dataset, as well as the users accessing them (ARNs) by running the list-ingestions CLI command [1] listed along with the describe-data-set [2] command:

$ aws quicksight describe-data-set --aws-account-id <AccountID> --data-set-id '<value>'
Please replace the '<value>' with the dataset-id of each dataset and check the value of 'ConsumedSpiceCapacityInBytes' in the output of the CLI command. 

To get the datasets IDs, you can list of all DataSets in QuickSight account, using list-data-sets CLI [3]

$ aws quicksight list-data-sets --aws-account-id 152558349946

If you prefer to use the Python SDK, sample code is below to retrieve data:

To list the datasets:

response = client.list_data_sets(
               AwsAccountId=aws_account_id,
               NextToken=next_token,
               MaxResults=100
)

To list the SPICE ingestions:

response = client.list_ingestions(  
               DataSetId = data_set_id,
               AwsAccountId=aws_account_id,
               MaxResults=100
)

[1] list-ingestions: https://docs.aws.amazon.com/cli/latest/reference/quicksight/list-ingestions.html#list-ingestions

[2] describe-data-set: https://docs.aws.amazon.com/cli/latest/reference/quicksight/describe-data-set.html

[3] list-data-set: https://docs.aws.amazon.com/cli/latest/reference/quicksight/list-data-sets.html

Other References:

You can also see the Aggregate SPICE metrics - https://docs.aws.amazon.com/quicksight/latest/user/monitoring-quicksight.html#aggregate-spice-metrics

And here is the link on how to view SPICE capacity and usage in an AWS Region - https://docs.aws.amazon.com/quicksight/latest/user/managing-spice-capacity.html#spice-current-capacity-and-usage

Finally, for QuickSight related questions, we have launched a fully dedicated community; in case you want to check there to see if there are further answer to your questions: https://community.amazonquicksight.com/