prevent changing password

hi guys i have an installation of WP running and want to offer my customers to write posts on my blog. for this i have set up a user called “guest” with a given password and the password is written in my office. so when someone comes he/she can post blog entries.
My question is now: can i prevent that the user changes the password. I want that just admins can change passwords and not subscribers, authors, etc. Is there a way i can bung in an IF somewhere..
thanks for your advice!