Hello @theshopnextdoor Welcome back to the Seller Community!
While Custom Amounts cannot be removed completely, you can definitely restrict the usage of it. In order to do this, you would have to go to your Team Permissions > Pick or created the permission level you would like to limit, and lastly head to Checkout and remove or uncheck the "Manually enter custom amounts" permission from that particular permission set.
Here's how that looks:
