Hi, welcome to the forum!
Either you can just delete the imported tracks (from library only), or you can find and delete the .mbl file, which will force MusicBee to recreate it on next start.
Before you do that, though, I suggest you read "Why are tracks from the same album shown separately in my library?" in the FAQ.
Regarding the mpeg files, I don't know how MusicBee normally handles them. Maybe someone else can comment on support for video files.