I use a password manager for large passwords. Today I've had to reset my password to something simple becuase the 'Reset Password' functionality doesnt recognise when I have pasted a password in from my password manager.
This also applies to changing passwords inside of the account pages. You add functionality to stop pasting of passwords. Why would you expect customers to type in their passwords manually? What if they're doing this with mobile phone and trying to make sure they get every character right when you don't allow a 'visibility' button?
I suspect something has happened in the past to cause this (not looking to far into the pas though). So I think you should review this and allow people to use password managers and pasting into password fields so that we can use more passwords and therefore increase security.
Theres another idea already out there and I think that you should use 2 factor authentication to secure accounts.