Personally I highly doubt that said app, or any other app for that matter which is downloaded from the Marketplace here is the cause of it. Everything that get submitted it here is thoroughly checked by the MP Moderators.
You don't need to do a fresh install imo, all you have to do is overwrite your forum files with those from the Ipb package that you can download from your Client Centre. I am assuming that you are running the latest version, if not it would be best to upgrade your forum asap. Doing that it will ensure that any infected core files, will be cleaned up automatically.
You mentioned Wordpress, are you using their latest version? What about any of their third party apps/plugins, are you using their latest versions as well?
What should be done imo, is to do a very though checkup of your server space for any backdoors that might have been left behind.
Also did you contact your host? You can aks them to check their logs around the time that the infection happened in the hopes that the culprit can be identified and be dealt with.