Query Based On 'does Not Exist' Condition

Let's say I have a table called 'forumtopics' with a field representing the topic author's username. I then have another table called 'ignorelist' which has two fields; one is an account number and the other a reference to the aforementioned author. There can be many ignored authors for any one account.

For any given account number, I'd like all the rows from the forumtopics table where there is no match for (account, author) held in the ignorelist table. Obviously it's easy to check for existence but can the opposite be done?

I run MySQL 4.0.17.