can developers with write access delete source code already pushed?

[Mike Friedrich]

You can configure branch restrictions in Bitbucket to avoid that, even if users have write permissions. This way you can protect your master,  integration and release branches. 

I would not restrict user branches.

/using-branch-permissions-776639807.html

I would not restrict user branches. 

See

[Seth]

Your link is only a partial link.

[Daniel Wester]

Yes. There's a blog post at https://developer.atlassian.com/blog/2015/04/force-with-lease/

[Fabio Racobaldo _Herzum_]

Hi @herbertww ,

yes, they can. If someone commit something, another user can overwrite these changes with a new commit. 

Fabio