It doesn't look like there is a way to manually purge data already recorded. If data stops flowing, the datacenter application and service entities will gradually fade away in Dynatrace.
The "fading" algorithm is based on data retention periods for metric resolutions, which are 14 days for 1-min, 28 days for 5-min, 400 days for 1-hour and 5 years for 1-day.
Say a software service A monitoring stopped a week ago. If I look at a list of software services today for last hour, I will see this software service, grayed out. If I look at the same list of software services, for last 1 hour, in a week from now (two weeks since monitoring stopped) - software service A won't be there. If I change time window then, to last 21 days, I will see it, grayed out. If I change the window to last 30 days - I will see it, regularly black, with data from last times it has been seen.
And so on. I hope it helps at least a bit.