You're running a Wordpress blog. Wordpress plugins are fairly commonly found to have vulnerabilities that could allow an attacker to gain unauthorized access.

Instead of deleting and restoring from backup, you need to find the way they're coming in and fix that. It would be like demolishing your house after someone stole the keys but leaving the locks the same when you rebuild.

You're best bet at finding how they got in is to look through your webserver logs. Any entries that look "odd" should be investigated. (usually Google is your friend for this)

If you have any specific questions about log entries, feel free to post them here.

If the problem is with WordPress, you should probably upgrade it, and all the plugins to the latest release. Make sure you have strong passwords as well. Depending on the what was vulnerable, your entire system could be compromised and you may need to format and reinstall to wipe out any backdoors. Some WordPress vulnerabilities allow attackers to execute remote code on your server which eventually leads to remote access.

Have you tried WP-scan that may put some light on any plugin that are outdated or have issue. There are also lots blogs that give some tips on secuing wordpress.