adam’s fix will mean backslashes will be stored in the database. Worse, on upgrade, his changes will be lost and there all the backslashes will be.
Better to do this as a plugin, and filtering content before it goes in the db.
Just write a plugin, upload it and activate it. Like this:
Plugin Name: Strip Slashes
add_filter('pre_topic_title', 'stripslashes', 40);
add_filter('pre_post', 'stripslashes', 40);
add_filter('bb_add_topic_tags', 'stripslashes', 40);