Yes, thankfully there is... but they don't make it easy to find the settings!
**This wont work via the chrome app so you'll have to use browser or the Mobage app**
From in-game Granblue go to My Page -> Top Page. Right at the bottom is the link to your Mobage profile, so click the one of the buttons (shouldn't matter which one) Click the menu button [メニュー] on top right Click settings [設定] on the bottom right of the pop-up
From here you will get a list of sub-menus. Need to go though several of them. (numbered from the top)
4:メール通知設定 You should see 2 button. click the bottom one [まとめ通知設定したい方] Check the box that says [受け取らない] Then his the button at the bottom [確認]
Go back to the menu list
5:メルマガ配信設定
You will either see one or two buttons. If you see only one that says [配信登録する], you don't need to do anything. If you see two, click the bottom one which says [配信停止する]
Go back to the menu list
6:各種ニュースメール配信設定
Should be 2 drop down listi. Set both of them as [送らない] Hit [設定]
Go back to the menu list
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I think that should be all you need. Depending on your device the main menu button might be in a different place so let me know if you're having trouble finding anything.
Not sure if any of the mods still support this community, but they should be aware that the forums are infested with bots and that their forum needs to be upgraded.