Spam and blocked senders
Moving an email to Spam also blocks its sender. Moving that email from Spam to Inbox marks it not spam and removes the sender from the blocked list.
Retention
Under Settings > Preferences choose 7 days, 14 days, 1 month or Never. The regular background cron permanently removes Spam older than the selected period.
Folder actions
Right-click, long-press or tap the three-dot control beside a folder to empty it to Trash, archive all mail or delete a custom folder. Non-empty folder deletion always asks for confirmation and moves its mail to Trash first.