You mean you want to remove rows from Flashback Data Archive that appear when you run:

select * from table as of (timestamp|scn) ...

?

If so, the short answer is no, you can't.

The data in the archive is read only, so you can't selectively remove it. That said, there are a couple of options to wipe old info

Purging data

You can remove all the data in FDA before a given point with:

alter flashback archive <arc> purge before (scn|timestamp) ...

Note there's a bug in this (19206785) so it doesn't work up to 12.1 :( It's fixed in 12.2.

Disabling flashback archive

Purging removes the history for all tables. If you only want to remove one you can by toggling flashback archive on and off:

alter table <tab> no flashback archive;
alter table <tab> flashback archive;