You can't easly ruin pointe shoes. I know that lots of people didn't believe what I do to pointe shoes to prepare them, or that the best pointe shoes (IMHO) have nails in them.
But one way to ruin them is breaking in them wrongly. I did it once, with a pair of Sansha Recital. But I've saved them!
Use can use this tutorial to re-new old pointe shoes that have the shanks too soft. Maybe you won't want to dance on your Premier at the Scala theatre with them, but they'll be good enough to exercise.
Please, note that I'm not a native English speaker, so forgive my mistakes (or gently tell me what I've to fix!).
one pair of badly broken in pointe shoes
I bought the stems on Sansha website, but some shoes are solds with them (one of the pair of Premiere I bought have them).
Step 1: Look If Your Shoe Have a Hidden Space
You should see the shank.
Some shoes have a pocket to put the stem in, like Premiere, some others have just a space, like Recital.
Step 2: What If the Shoe Doesn't Have a Hidden Space
If the shoes don't have the nails, just tear the glue and re-glue them after putting the stems. If they have the nails, you should open the nails and then hammer them back.
In the photo you can see a ReArt Smezzata. I used them for a while and the sole of the right one broke, so I totally broke them, put in the stems and re glue them.
Step 3: Put the Stem In
Once the stem is all in, you can close the pocket or reglue the soles.
Step 4: Break in Them
The red line in the photo is where the stem will be. You can break the shoe where it finishes. The stem is flexbile and it will give your foot more support because it will make the shoe sole harder.
Put them on and break in them, time time in the right way :D You can look at this: