Polish hacker posts to my blog

I visited my blog today after a really long time of inactivity and
> found some one has gotten in and posted things in Polish.

When I deleted the posts – updated my plug ins and my whole blog fell apart.

Visited blog two days later and theme was back up with another Polish post – Amster8881 is his name. Deleted his post and again theme collapsed.

This time I went in and deleted him from registered users and any other admin type – and changed the password.

Probably not enough to keep him out?

Help please.