Set that same permission to No for the Registered Users group in the forums where they shouldn't be able to lock their own topics.
Can't change a forum:
SQL ERROR [ mysql4 ]
Unknown column 'forum_post_tpl' in 'field list' 
UPDATE phpbb_forums SET parent_id = 3, forum_type = 1, forum_status = 0, forum_parents = '', forum_name = 'Ideas and Bugs', forum_link = '', forum_desc = 'Post here your Ideas and Bugs about the forum!', forum_desc_uid = '', forum_desc_options = 7, forum_desc_bitfield = '', forum_post_tpl = '', forum_rules = 'Only ideas and bugs about the forum, [b:2xeb188l]NOT[/b:2xeb188l] of the game!', forum_rules_uid = '2xeb188l', forum_rules_options = 7, forum_rules_bitfield = 'QA==', forum_rules_link = '', forum_image = '', forum_style = 0, display_subforum_list = 1, display_on_index = 0, forum_topics_per_page = 0, enable_indexing = 1, enable_icons = 1, enable_prune = 0, prune_days = 7, prune_viewed = 7, prune_freq = 1, forum_flags = 48 WHERE forum_id = 15