Mitigating the slippery-slope of day-after-day deferral could be priced in, so each deferral gets more expensive. Or a limited-use mechanism like @martyh suggested. But here's the thing:
@chelsea's point is apt; informal mechanisms exist. Folks put all sorts of things in their fine print (recorded or otherwise) that could let themselves off the hook today. The best examples require actual progress toward the goal, like @malcolm's incremental progress toward the next integer value. Some of the worst ones attempt to play catch-up and aren't written down, so it's hard for the user to apply their own work-around consistently.
That's not ideal, but it's also not a blatant abandonment of beeminding. An alarm clock works best if you get up when it goes off. Hitting snooze is a slippery slope, so much so that habitual snoozers don't even see it as a problem. "I press this button 3 times, and then I get up." It makes an alarm clock less effective, but not useless.
Start with the assumption that people are flawed yet heroic; they're beeminding things precisely because they wouldn't get done otherwise. Even if they hit snooze a few times, they'll still get more done with our help than without it. Less effective, but not useless.
Official mechanisms supplant informal mechanisms, because: path-of-least-resistance and defaults. Informal mechanisms spring up to work around inflexibility, because: Ashby's law.
Before we had Take a Break, informal mechanisms existed; some people remembered to flatten and unflatten their roads, others emailed support, others entered a large datapoint to create a week off. Now that there's an obvious and easy official mechanism, there's less use of informal mechanisms.
This isn't about appeasing cheaters. It's about supporting heroic efforts to stay within self-imposed rules. And because people are flawed, it's also about giving them the tools to help them to clamp down on themselves, e.g. by closing windows of temptation. And that enables their own personal kaizen.
We've all got slowly better at creating the right goals, with more appropriate fine print and sustainable slopes. But it's slow and incremental, and it relies on us making the best use of Beeminder that we're able, based on where we are right now. Some of that use isn't going to be ideal, or as effective as we will learn to make it, but it's not useless. Flawed, yet heroic.