There are a couple of ways to go about it.
1. Instead of "Replying" in the normal manner, you can simply respond to the original post in the thread, which will allow for a reponse to be posted but will place the post on the left side of the page, as if it were in response to something else. It still appears like you are responding correctly.
2. You can also "work around" the "No further replies to this post will be accepted" message by looking in the address bar at the top of the screen, change the 5208 number in the address to 5224, then press the "Go" button in the address bar. The standard "Reply" window will load and will allow you to continue responding.
Hope this helps, too.
Grif