"During which I couldn't do anything else" is stronger than I thought at first, and suggests some kind of windowing system (input) lock. If you just wanted to introduce latency of reward, a proxy for the web page loading would be fine. I don't know which proxies have that as a standard feature.
I think a reminder (extra step to get the reward, which you can decline to execute) is good. Mechanisms that make it actually impossible to view a page or run a program (short of reboot) are stronger than I'd like or imagine I need.
The alt text of today's xkcd reads:
Anyone have ideas about how to implement this? On Firefox one can always use LeechBlock. On *nix systems there would be a number of ways of implementing this, but not all of us use that OS or have the necessary savvy.
(I'm kind of surprised no one's made a discussion post on this yet.)