Your servers that get abuse reports certainly need more special attention. Especially authentication attention, attention to the software used, and attention to host filtering.
Make sure that only certain people can access your server using certain IPs. This way, security is better maintained.
Make sure the software package is always up-to-date and use the stable release version only.
For the web server application you use, make sure the CMS or script used is one that has minimal vulnerability.
If you have database authentication whether it's MySQL, PostgreSQL, Microsoft SQL, or anything else we recommend not listening to open ports to the public. Make sure to filter only certain IP addresses that can access the port.
If you have FTP (File Transfer Protocol) authentication, make sure to use a difficult username and a very complex password.
For SSH authentication, it is highly recommended to use a very complex password, and user login is limited to a specific IP address.