how to remove all posts by a user at once
 

I need a better control
I want to be able to delete all posts by a user at once

how to do it?

Search for Spam Decimator ... Im pretty sure that this one does what you want. I think there is another called 1 touch spam that does this as well.

Can't you go to Threads & Posts > Prune and at the bottom (where it says "Prune by Username Note: This includes individual posts made by the specified user") enter the username, select All Forums, and hit Prune Threads.

Yah, but then make sure you rebuild thread information, otherwise, the thread counts will be off.