Since OO 10.51, it has been possible to purge flow execution data of a specific flow by calling the /executions REST API. For example, sending a DELETE http request like below to your Central purges flow execution data of the flow with UUID 79e6def8-435a-4238-8c92-b168d82f24ed which ended before 2018-08-29 6:38:39 360 UTC.
Please note that the 'endedBefore' request parameter is mandatory and its value must be an epoch(unix) timestamp in milliseconds.
Also if you use get an epoch(unix) timestamp from the 'Library/Utility Operations/Date and Time/Get Epoch Time' operation in a Base CP, the operation returns a timestamp not in milliseconds but in seconds.
See the following manual for more details about the REST API.