Securing my forum from hackers and exploiters

My forums has been getting hacked too much. Probably twice a week we have been getting hacked. We then found shells in our public_html folder and removed as many as possible. It was not enough today our forum files were all changed to "FUCK YOU" and we were defaced. We are installing a fresh mybb download and are reinstalling all our plugins. We think that a plugin we installed was what caused us to get hacked. This plugin is a live chat plugin and is the life of our forum, we would need this patched. We also need our encryption methods changed to something more secure. I want to make sure my forum is secure.