- Forcibly combine posts sequential posts by the same user.
- Completely disallow sequential posts by the same user.
If sequential replies are very short they are better to be completely disallowed because such a message stream converts forum to chat. If they are not short they could be forcibly combined because it simplifies reading and there are use cases where it is really convenient, for example, when someone replies by email.
And also there is support kind of topics like Q&A where even non-sequential messages can be combined to increase content readability for later readers.