The logins are currently referenced in existing deployment profiles.
Find and remove the login references from the existing deployment packages.These can be found through the following steps:
1) Run the following query against the CDA database (where 'loginObjectName' is the name of the login to be deleted):
select LoginId from LoginObject where Name = 'loginObjectName'
This should return a numerical value, for example 123.
2) Run the following query against the CDA database using the value generated from the above query:
select * from DeplProfile where LoginId = 123
3) The values under the 'Name' column indicate the Deployment Profiles where the login is still used.
4) Remove the login from the Deployment Profiles. The delete button will now be available for the login.