OK, I've been testing heaps and am ready to clean up.

Along with my valid posts and valid users,
there are heaps of invalid posts and heaps of invalid users.

I'd like to be able to batch process removal of specific users and/or their posts.

I've managed other BBs that have this but can't seem to find it in miniBB.

Any positive constructive assistance would be greatly appreciated.

I'm not sure how could it be possible to batch process of removal posts. On which selection criterias should it be based?
In which exactly "other BBs" did you see this feature?

In miniBB you can remove multiple posts of a certain topic at once (under Manual, check for setting up the $enableGroupMsgDelete value under setup_options.php), or install Multiple Topics Action add-on to move/delete multiple topics at once.

For miniBB, there are few add-ons to prevent unwanted posts (Anti-Guest add-on free, Pre-moderation add-on paid) and unwanted registrations (f.e. under Manual, check for $closeRegister option under setup_options.php).

Removing multiple users is available under the Administration panel. You can search members for certain criterias, and then remove them directly on the results page.

Customizations are available on request.

I should be able to select on any criteria.

Once the results have been displayed,
a check box against each row is ticked.
Or a Select All button or a Select None button.

Then a Delete All button and a Delete All Including Posts button or a tick box for Including posts.

Then loop thru each of the ticked items and perform the action based upon which button is pressed.

Removing multiple users one at a time is not what I'm after.

Now I'm wondering if it would be easier to just make another forum and copy n paste or perhaps use a mySQL admin and delete by hand.

But now I'm thinking what happens when I want to remove a forum or perhaps change its subject. Don't want to get stuck doing it by hand again.

I already replied above - it's all about ticked boxes. It's possible if you follow my instructions.
Ticked boxes will appear near each message, each topic or each user depending on what are you are in. It's that simple.

It's not about "batch process" after all. "Batch" is a phrase used for automated processes and can't be applied to this operation.

Removing the forum is possible from the admin panel.

For me it seems, you need to study software's documentation at first, before concluding what is available/possible and what not. Like in any other case.