To exclude forums in the New Posts button, you may add "&exclude=xx" or "&exclude=xx,yy,zz" (separate the forumids by commas) to the end of the New Posts url... ie. "/forums/search.php?do=getnew&exclude=xx" This would normally be in the navbar. I think it occurs in a couple of places there, so you will need to edit each of those urls.
This is the old version and is the old code, not 3.8.1 compatible. This is the code in my navbar for today posts: