Do you want to delete your notes or prevent people from sending you notes? You can delete your notes as goatgirl described. You can also prevent people from sending you notes by going to your Settings page. Then click Privacy. There you can change the drop-down that says, "Send and view my notes" to Nobody or Only My Friends.
If your friends are sending other people notes, these will show up in your Friends' Activities. There's no way to remove those.
Thanks,
MedHelp
To delete a "Note" from your user profile page (My MedHelp), click the “See All” link in your Notes box. Then click the “Delete” link for each note you wish to delete. Not sure there is a way to remove your friend's activities from your home page. Hope this helps.