On your iPad, go to Settings → General → Storage & iCloud Usage → Manage Storage → Music.
From there, you can either Edit or directly swipe left on any artist to delete all their songs. If you just want to delete individual albums or songs, you can drill down by tapping an artist or album, respectively, then swipe left to delete each item one by one.
If you want to go thermonuclear, you can simply swipe left on All Songs. Do note that this operation takes some time to complete, up to several minutes. So just let iOS do its work, and soon enough you should have your storage back.
Once you’re done, in order to keep songs from being downloaded in the future, go back to Settings → iTunes & App Store, then toggle Music Off under AUTOMATIC DOWNLOADS.