Can I claim an account with no activity?
The username you want is taken. You visit the account profile and see no account activity, can you claim that account? No, you can't. And this is why...
Bitbucket offers unlimited free private repositories. Many users take advantage of these free repos. These users create a account, join a team, and work 100% on private team repositories. When users work like this, you can visit their profile pages and you may not see any activity for them. As a matter of fact, you may only see that they've signed up. Yet these users are hard at work in stealth mode.
Additionally, Bitbucket allow users to choose their own username. The first person to choose a username gets it. We won't ever give away another user's chosen username. It is theirs to keep for as long as their account exists.
In short, the answer is no. We won't give away another user's account for any reason.