Assigning new empty collection is probably the fastest way. Remove all Rows if you need to preserve fields, but you can just keep an empty copy.
p.s. My limited investigation tells me that you need to clean up memory in VBOs too, otherwise it is not released after process end until you initialize that VBO again in another process. None of the default VBOs do that. For example, Append rows in Collection manipulation will leave behind two copies of the data in memory. If another process runs that never uses that VBO - that memory seems to stay unavailable (even after GC.Collect). If you change Collection manipulation to empty all collections on clean up (or with an action) - memory is reclaimed - resource pc process returns back to where it started on memory consumption.
