I accidentally found another way around this... Over the last weekend I somehow enabled my 4G radio. When I opened my browser I was redirected to https://deviceselfservice.sprint.com...lcome.actiob.c
... (URL shortened) So I did a little googling and found that you can get one day of free service on 3G/4G by using the promo code Free Day (it is case sensitive)... After a reboot, my tablet was able to activate with no issues and since the trial expiration, I haven't been bugged about it since.
So... If you aren't rooted and don't feel like taking the plunge, you can get rid of the nag screen this way.