bbPress.org

Yeah, but in forums like phpBB, if you close a topic you are locking the topic. This is indicated to the user by a icon. I added some styling which I had gleaned from the internet:

#bbpress-forums .status-closed > li.bbp-topic-title > a.bbp-topic-permalink:before {
	content: "[Locked]";
	text-shadow: 1px 1px 0 #ffffff;
	color: #ff0000;
	margin-right: 5px;
	font-size: 16pt !important;
}

This gives me a lock symbol but the look and feel of this symbol is not as nice as the paper clip which is visible for topics with attachments (using the GD Attachments plugin).