Unfortunately I cannot claim credit for this really simple and easily performed fix.
I found it on an old forum thread. (Original forum thread on How to Mend It: http://www.howtomendit.com/answers.php?id=199139)
All the links to the explanatory pictures posted over the years no longer work.
From reading the descriptions I pieced it together. Not that that was too difficult, but there is no standard way to explain oven door hinges!
The reason the problem comes about is because either the springs tension in the hinges to keep it closed are no longer good enough , or the lip (this lip will become more apparently shortly) on which the arms of the hinge that extend into the oven rests, wears down with use.
While what I am about to describe works on Smeg oven doors, this fix apparently works for other brands of doors (such as Ilve) which have a similar mechanism.
Firstly remove the door.
1)Open fully. All the way down! Undo small levers (catches) between the arms on both sides. If they will not unhook, gently push down on the door a bit and see if that helps.
2)Half close door (45 degrees) and pull the door out.
This youtube video explains door removal as part of hinge replacement.
And the problem he has with the door at the beginning of the video (0:08) is the one this will fix.
The fix entails using tinfoil. I used standard thickness baking tinfoil. Nothing special.
Tear off strip about 4 cm (you will probably need less) The roll I used was about 20 cm across, so a 20cm x 4cm strip.
Fold the foil in half repeatedly until its about 6-8 layers thick.
Cut this into two strips about a centimeter long and just wide enough to fit into the width of the opening into which the hinge arms of the door go on the oven.
Fold in half and place over the metal lip at the lower end of the opening on the oven. Press it down snugly with a tool so that it seats well and will not be easily dislodged.
Repeat for the other side.
Replace the door. The little notch on the lower arm of the hinge on the door (2:46 in the video) fits/rests over the metal lip in the oven opening.
The idea is that there will now be multiple layers of tinfoil between the notch and the lip like this.
Don’t forget to close the levers (“catches” in the video). If the little levers won’t engage fully into the locked position, make sure the door is fully open (3:10 in the vid). I had this issue.
If this fix doesn’t work because your hinges are truly broken and not just slowly failing, it appears that replacing them is not too difficult if you have the spare parts. I might need to do this in the future.
Good luck!