2) Timethief linked to the relevant Support doc: you probably didn't read it carefully enough. The changes you make in Settings > Discussion become your new defaults, i.e. they apply to posts/pages you create after you've made those changes: they don't apply to already published posts/pages. To disable commenting on an already published page such as the one you linked to, go to Pages > All Pages, hover over the page title, click Quick Edit, uncheck the option " Allow Comments", click Update.

http://stivershamblen.com/rsvp/

I've read elsewhere (http://wordpress.org/support/topic/leave-a-comment-link-wont-disappear) that this is a glitch with WordPress that needs to be corrected through the PHP file, but I don't know how to access and change that, so I am hoping to find a CSS solution.

Comments on all pages and posts on every WordPress blog are enabled by default so that means you disabled them on the posts you refer to.

This page > Settings > Discussion
Discussion settings is where you set up defaults for comments.
http://en.support.wordpress.com/settings/discussion-settings/

We can also override the default settings and enable or disable future comments on any individual post or page.
http://en.support.wordpress.com/enable-disable-comments/

Note: By default, new blogs on WordPress.com will have the Discussion hidden. You can reveal it by clicking on Screen Options http://en.support.wordpress.com/screen-options/ on the top right corner of your Admin page, and checking the box that says Discussion so the module is included. http://en.support.wordpress.com/modules/#list-of-modules

I'm also looking for CSS code to change the appearance of my RSVP form -- unbold the field headings and make some of the text boxes smaller, for example.

Help?