I guess the real question here is why all users are using the same username. If it's through an application, presumably there's some type of login or other authentication mechanism, so you could use that to log the activity each person does to build tracking into the system.

If it's through SSMS, then everyone should be connecting with their own login, preferably a Windows login. Then again, if they're connecting via SSMS and you have a recurring accidental deletion problem, maybe they shouldn't be.

I don't know the exact details of your situation, so take that into account when considering my advice.