If the logs where setup for another AWS service they will normally have the service name in the log group name which you can view in Cloudwatch logs to help figure out where they are coming from. If you have multiple log groups, https://aws.amazon.com/premiumsupport/knowledge-center/cloudwatch-logs-bill-increase/ can be used to figure out which ones are causing the cost increase and de-active the relevant settings to prevents the logs being sent to Cloudwatch.
If the logs are needed its worth adding a retention time to each Cloudwatch group as by default the logs are kept indefinitely, doing so will help keep your costs for "Log Data Archive for Amazon Cloudwatch" from growing more than they need to.
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