We need to speak about it more. I don't understand what you are about :)
Start from here:
- who is a "new member"? how to recognize him?
- how these topics are set up? how to recognize them?
- non-staff members - who are they?

Hi Paul.

Sorry I didn't make myself very clear before.

New member = registered users who feel the need to make pointless replies (e.g. I agree/Thanks/Cool/Me, too!) to lots of topics each day. From experience, these users tend to be young in age and ignorant of forum etiquette.

How these topics are set up / How to recognise them? = topics in a members-only forum.

Non-staff members = regular registered users (as opposed to staff who are registered users with moderating/admin privileges).

Your "daily message quota per IP" solution is very useful for containing/minimising damage caused by aggressive spammers and flooders. Can this solution be extended to control the number of topics members can post replies to in a day?
For example: currently, if we set the daily message quota to 10 messages per IP, a user can either use up the daily quota by ...
a) starting 10 new topics
or
b) making 10 replies to one or more existing topics started by others.
The idea is to modify the solution so that it will turn (b) above to "making 10 replies to a maximum of 5 existing topics started by others".
Would this be a sensible approach to help keep users from going on a "fox-in-a-hen-house" spree of pointless replies to a load of old threads in one go?

Any suggestions/advice will be appreciated. Thanks and cheers -
mb

"making 10 replies to a maximum of 5 existing topics started by others" - ok, if this is all about your request, if I'm getting this correctly, the script should calculate all unique topics the user has posted messages in, before making a conclusion on the next message allowed. And if this amount exceeds 5, then the user can post a new topic anymore? Is it what you would like to request?

So say if I want this to work on 'guests'/non-registered members I'd set it as: $user_id<0

Nope, the condition for guests would be just
if($user_id==0)
There are no negative user IDs in miniBB.