How do I block outgoing mail to a few external addresses?

A mail flow rule is a good fit. Set it to apply when the recipient is outside your organization and matches one of the addresses, then reject the message with a short explanation. That keeps the rule aimed at outbound mail rather than messages coming in from those addresses.

The recipient picker can be awkward for addresses that are not in your directory. If those addresses are available as mail contacts, you can select them as recipients. Otherwise, use a recipient-address matching condition for the external addresses, along with the condition that the recipient is outside the organization.

Be precise with the matches. Avoid broad partial-text matching, and test the rule with both a blocked address and a similar address that should still be allowed.

If this restriction should apply only to certain senders, add a sender condition too. A mail-enabled group can make that easier to maintain if the people covered by the restriction change over time. The key is to keep the external-recipient condition in the rule so it does not block incoming messages.