While I was not advocating anything as bizarre as a per minute penalty (I would screw that contract up and throw it away as well), I would still say that e.g. reducing PO by 10% of project value per 24 hours late would be valid.
I think this would be a valid approach. It would compensate you and the end customer for the delay in really urgent jobs (although in many cases it would not solve the problem if the text was needed for some meeting or sales presentation at a fixed date and time), and it would encourage the translator to better stage the job and plan for it.
"I didn't realise the job was so big" or "I have decided the rate is too low for me to complete"
These really look like poor excuses and nothing you can do (except discontinuing the relationship of course) will solve the problem. But just in case they are not excuses, maybe a bit of education for some of your translators could be useful. You might want to write a simple article with recommendations for new translators (and old vendors of yours where needed) on things like:
1. Keeping a simple list of tasks with their delivery dates (and times if appropriate), volume of new words (and different match ranges if a translation memory is used) and details about the end customer and the PM ordering the job. Classifying email in folders might work for some people, but a clear list always works in my opinion.
2. How to estimate the real time a translation will take (with examples for different types of documents, documents with or without use of a TM, documents with or without tags in them...).
3. How to read your PO, what parts of it should be read upon receiving a job and why is it important that those details are read immediately.