No we don't have any options for blocking your data from specific people. Everything you enter into Fuelly (except your email address) is public, even to non-members, so be sure you're comfortable with anyone seeing it. Keep in mind that you're not required to associated your Fuelly identity with your real life identity. But if you're still not comfortable with the data you're sharing here you'll probably want to switch to a private mileage tracker.
Yeah, just to expand on what pb said, we don't offer private accounts because the data is all aggregated publicly, and we didn't want to create an issue where we are showing data publicly from private users in the averages but people browsing car information can't get to all the data.