Hi, Brian. This isn't built into the plugin, but is possible with some added code in your themes functions.php (or a companion plugin).
There is some code for our support members here:
I can give you the gist of it:
* Hook into WP's template_redirect hook
* use the pmpro_hasMembershipLevel() function to check if the user is a member
* be sure to also exclude certain "okay" pages (homepage, levels page, checkout)
* use the wp_redirect() function to redirect to the checkout page or wherever you want non-members to go
Hope this helps.