when you click on "show unred posts since last visit" (at the top) it comes up with a list. You can click on only the topics you want to read. When you're done and that list is showing there is an option by it that says "mark all messages as read" (this option also shows up on the main board)
I can't see a way on this forum to "ignore" a thread or board, however. Sorry, Liz - I looked! I use the above technique if needed.