In fact, it is code. What has most likely happened is that the person has prepared their post in Microsoft Word, then copied it into the forum. The “code” is all the HTML commands to make your text look nice in Word.
The solution is: don’t!
Either type your text directly into the forum reply box (which has some basic formatting buttons if you type in the Visual tab, rather than the Text one) or use a plain text editor like Notepad.
Sometimes, if I notice these I will edit the comment and remove the code, but I don’t always see them.