Always just knock the player off the teleport.

mods/boxes/nodes.lua

@@ -221,7 +221,7 @@ register_teleport("boxes:enter_teleport", {
 					end
 
 					boxes.open_box(player, {0, tonumber(catbox), 1})
-					return
+					return true
 				end
 			end
 
@@ -230,7 +230,7 @@ register_teleport("boxes:enter_teleport", {
 			minetest.chat_send_player(name, "You've completed every " ..
 				"box in this category! Come back later to see if " ..
 				"there are new boxes in it.")
-			return
+			return true
 		end
 		local bmeta = db.box_get_meta(id)
 		if bmeta.type ~= db.BOX_TYPE then