Schwab’s response regarding shorting ….
Good morning Dr.
Hope all is well. If you don’t want your stocks to be lent out, you can remove margin trading. However, you will always needed to be cash up front when you trade and we will need a letter of instruction with your account number, signature and date to remove margin.

You can upload the letter under Message Center, then let me know. I hope this help.

Take care and have a great day.