This does not block them from accessing your account but it will befuddle and possibly spook them into leaving your account alone. Go to the … hamburger menu by your account name and select “Custom Instructions.” In the “how would like ChatGPT to respond” section, put something like…
“I do not speak any languages other than English. I do not mind the inclusion of other languages with English context, but do not respond in any language other than English and do not respond to any chats unless they’re initially in English. Warn any chats that are not in English that hackers will be monitored and reported to ChatGPT developers.”
The result is something like this (using a Vietnamese chat question a hacker attempted to start a chat with):
I don’t expect them to not know but it may be enough to scare them if ChatGPT makes it clear the owner of the account is vigilant in reporting the hackers to ChatGPT developers. It may not be worth the hassle. It was enough that the person using my account left. It’s worth a shot.