This seems to be a common event here in Second Life. Someone gets an object and then it "deletes all your inventory" or hijacks your account and "steals your linden dollars"
Accepting Landmarks and Notecards are generally safe, but the problem comes when you receive and rez an actual object.
First, let me be clear that I have researched this several times and it always comes up in topics on various SL forums.
You generally have to grant the object permission in order for it to do "anything” The problem is that residents are always quick to click yes without realizing the possible consequences.
Sound advice is to never accept objects from strangers and always verify who the person is that is sending it. The creators name is usually listed in the pop up or you can check the object properties.
The bottom line is if you have any doubt, then do not accept it. If the person is known to you then you may want to send that person an IM about the object first.
The same rules apply in first life so why should it be different in second life. Avoid paranoia but when in doubt, exercise a bit of caution.This is similar to clicking on an unknown or unsolicited link in an email or even executable (.exe) file you may receive. Always know and check the source carefully.