Hmm, if I’m understanding what you want correctly, not really. The cumulative goals are pretty key to Beeminder, and other types of goals can technically be created… but they’re not really supported.
For instance, I have a time-based goal that is non-cumulative for not logging too many work hours per day, /clockoff. However, when I derail it can do add things to the graph, and setting breaks on it can be tricky as well. This is fine because obviously I work with Beeminder’s graphs every day and know exactly how they work, but for someone using Beeminder more casually I’m not sure it’s a good idea, because support won’t do special tweaks to your graph for you – it’s a very self-serve sort of thing.
If you did want to go down that route, you’d need to have a premium account (beeplus or beemium level) and make a custom goal.
To prevent you being able to accumulate extra time, you can use ratcheting (or autoratcheting if you have premium), but that won’t prevent you having to make up for a deficit – there isn’t a way that I can think of to get around that.