You may wish to check that your og:title tag on your community exists and has not been customized and that you're on the latest release. The og:title tag, which Facebook should be referencing, does not include the forum name by default.
Here is Facebook's debugger tool with this URL:
Following up: it looks like you're using your mobile's share tool or otherwise. The mobile OS is likely just grabbing the HTML title tag which does include the forum name. To remove this, you would need to customize your HTML title tag as there is not a setting to remove this in the core software.