Ok, I haven't tested it but something like this would be how you do it.

// add this to mysite/_config.php
DataObject::add_extension('BlogEntry', 'CustomBlogEntry');

// put this code in mysite/code/CustomBlogEntry.php
<?php

class CustomBlogEntry extends DataObjectDecorator {

public function updateCMSFields(FieldSet &$fields) {
   $fields->removeFieldFromTab('Root','LeftSidebar'));          
   $fields->removeFieldFromTab('Root','RightSidebar')); 
}
}

Hi Willr,

I tried the exact same thing shortly after I posted. I've got just what you posted (minus the extra closing brackets on the two removefieldfromtab lines), and it's still not working. I am very confused now.

Perhaps try putting a die() statement in that function. Does it even get called?.

I tried that too, and the function is indeed being called.

Now, my tabs for "Left Sidebar" and "Right Sidebar" were actually created on the Page datatype, which BlogEntry extends. That shouldn't matter, should it?

Well the first thing is to work out why that decorator isn't being applied. I'm guessing you're done the usual admin/?flush=1 to flush the cache?

I've done a flush on both /admin/ and on /dev/build/

The guys on IRC are stumped too. :(

Thanks for you help with that Willr, I managed to get it working with Subclassing the blog module in the end. (Had quite lot of changes to make.)