When I delete emails they just come back again as unread, I now have over 1000. How do I delete?

When I delete read emails they will initially delete but then download again as unread.  I now have 1000 emails and periodically the whole lot seems to download again, this takes time and is making my mail almost unusable.  The problem has been going on for almost 4 weeks.  I have contacted the helpline who, on two occasions, have promised a resolution within 48 hours.