Okay to clarify--just quoted the first person who responded but I guess I am clarifying for many
First I just want to expand on my earlier post. I don't think I would punish a younger child (6 certainly falls into that category) for slamming the door OR barricading it IF IT WAS THE FIRST TIME IT HAD EVER HAPPENED. I think most kids would not think about it in terms of possibly damaging the house or being a safety issue if it were not pointed out to them--especially if they are in an emotional upheaval at that moment. Assuming this was a first, in the OPs situation I would ignore it in the moment (figuring if the 6 year old could move the toy chest in front of the door by herself, then I should be able to push it out of the way with the door if I needed to in the event of an emergency and knowing that my child were not in a state to handle learning new info well). Later on (later that day or the next) once DD was composed I would approach it something like this:
Hey DD, we need to talk about what happened when you were upset about the computer this morning. Whenever you feel yourself losing control it is a good idea to ask to discuss it later and go to your room pronto--if you lose control before you do that I will send you like I did this morning. I can understand you needing time alone to get control of yourself, but you CANNOT slam the door or barricade it and here is why . . . If that happens again X will happen. I am, however, willing to leave you alone unless I have some reason to believe you or something else is in danger--you need to come up with an appropriate way to let me know you want to be left alone though. What ways can you think of? Let's decide on one now.
Then, if the door were slammed again I would remove the door (without saying a word and as the tantrum continued) and if it were barricaded with that toy box I would be more likely to take the box and it's contents as that would sting more and be just as appropriate.
As far as the comments people have made: yelling AT parents--no not okay. Going to your room and letting out some of your frustration by yelling at the walls or a stuffed animal? Sure. I would much rather that than a child with a lot of pent up anger inside. MANY MANY people (myself included) never have that kind of intense angry reaction to frustration, but some do--especially when they are little and do not have as many options available to work it out. I also think it is a good idea to talk about it later, in a calm moment and come up with better ways to channel the frustration during a melt down. DS11 rarely gets upset by anything. He is very easy going and the biggest thing I hear about him from every adult who works with him is how polite and sweet he is. However, when he does lose it he LOSES IT

NOW he is old enough to get on his bike and ride hard to let his feelings out, but as a younger kid he just needed to hang out in his room alone and growl for about 30 minutes

As long as he was in his own room (and HE has always been the type to put himself there) why do I care if he is growling if it is not directed at anyone? I would think yelling would be the same.
It is lovely that so many of you can keep a kid from ever having any type of a melt down past the age of two. Other kids, in spite of having never had an adult give into a tantrum still lose it once in a while. it doesn't not make them bad kids or their parents bad parents. It just makes them more emotionally intense people than others. They need to be taught how to handle that intensity but you cannot simply deny that it is there.