Google has enough private data, so exept removing all incoming mail to the computer/e-mail client ( we can also remove sent e-mail. Google possibly intentionaly does not have option to do it. We have to setup new filter like this:

Go to Gmail settings, Filters, select all and export. Then either delete all filters or just append new one that contains rule "in:sent" in field for matching words, action "delete".

In the filter list i see the result as:
Matches: in:sent
Action: Delete

it appears to be working, maybe if you leave your previous rules, it can put some mail to Trash folder and POP3 client can not obtain such mail. Yet gmail will remove trashed mail older 30 days automatically.

Following approach WAS PROBLEMATIC (sent mail was deleted, but it was deleting(trashing) also some received mail):
1. in Gmail account is the search field on the top of the page, at the end of the field is arrow, click it and fill in following:
from: [email protected] (in case you are sending e-mail from multiple aliases, you can use: [email protected] OR [email protected])
search in: Sent
2. click Search button to verify it found the e-mails you want to permanently delete now and always in future
3. if it displayed correct e-mails, then click arrow button again (at the end of search field) and then click button to create filter.
4. select option to delete matching e-mails and possibly tick to delete X matching messages (if you want to permanently delete it). submit.

After a while after refreshing page, you should see Sent folder have been emptied. And hopefully no new sent mail will be retained in it.