Entry Security?

Previously, a Chinese website kept inserting a malware script into the footer.php of the active theme at our website: https://www.sophisticatesblackhairstyles.com/

I’ve installed several security plugins and this seemed to have worked for about a month.

Today, I logged in to find that they’ve inserted the same script into our most recent post entry. Do you have any suggestions on how to prevent this from happening?

Thank you.