The process differs slightly depending on if you are using iOS or Android.
On Android, you can turn them off completely so you do not receive any notifications at all about transfers, if you want to do that you will need to change your default notifications, instructions for that are here:
Or you can remove transfer notifications on a team by team basis if you only want to stop receiving them for one specific team.
To do that, navigate to that team's page, press on the bell in the top right hand corner and then select 'Customise' from the list. From there you can turn off the 'Transfer Notifications' slider. Here is a video showing how to do it:
On iOS you can only turn them on and off on a team by team basis. It is a similar process to on Android.
You navigate to the page of the team you no longer want notifications for, press on notifications from the list on their team page and then turn off the 'Transfer News' slider.
Here is a video of how to do it: