How to fix it?!
Can you show us a screenshot please or provide a link to the board.
Use a fixed width in .body class in global.css
@iGeorge: He just provided you a screen shot, at least pay attention.
@OP: It's a known bug of the MyBB Post Parser. However, I just looked at your forum and I'm fairly certain you didn't make that theme.
When i add fixed width - it's look like that:
HELP ME!
Ref! .
Type actual words and you won't have that problem.