If you've used the #movetodon webapp to help move from Twitter to Mastodon, consider revoking it's read-privileges to your accounts on both twitter and Mastodon.

On Mastodon, go to Preferences, then Account, then Authorized Apps, and then click "Revoke" in the top-right of the Movetodon box.

On Twitter, click "More" on the left side, then Settings and Support, then Settings and Privacy. Now click "Security and account access", then "Apps and sessions", then "Connected apps". Click Movetodon, and then click "Revoke app permissions".