There is no forum setting or code within Nabble for this.
Rather it is the part of the default stylesheet that (nearly) all browsers use that underlines links.
If you wish to have your forum adopt a non-standard appearance you will need to access your forum's home page and add the appropriate CSS code in the edit box at:
Options > Application > Change appearance > CSS
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.