The best solution is to make a backup before importing any data... Now to make a persistent delete of the records you have to play with the database or use code below in EpesiShell (/admin/ -> Run php command).
I can't guarantee that this method will not make any harm. If any user added any type of record (meeting, task), that involve one of records that are going to be deleted, then your EPESI installation will be broken.
$rs = new RBO_RecordsetAccessor('contact');
$records = $rs->get_records(array('>:Created_on' => '2013-05-12 0:00:00'));
foreach ($records as $r) {
$rs->delete_record($r->id, true);
}
Of course adjust recordset and created_on date.
Regards,
Adam