Ok, you should use <divs> for layout and table just for data (checkout bootstrap). But, since for that you would need to redesign the whole thing, here's your solution; it is pretty simple and you just need to add two things in two different sections.

Add: "table-layout:fixed;" to the style attribute of the table that represents each post in a thread.See how the code should look below.



AND also, you need to add: "word-wrap: break-word; overflow: hidden;" to the style attribute of the column where posts are displayed. See below how it should look.



Please feel free to let me know if you have any questions.

Edit
Click on the images to see full size.