OK, so this is a little embarrassing.
I noticed whilst looking for the handle this little snippet in the plugin code
if ( wpforms_setting( 'disable-css', '1' ) == '1' ) {
As it turns out, this refers to a setting you can utilise. You can simply do this through the plugin settings.
In your dashboard go to WPForms > Settings and under the General heading select ‘No Styling’, and hey presto, job done.
Sometimes things are a little easier than you assume.