Salesforce users can have a repetitive string as a password. Can I as a admin do something about it? The main reason is to force the users to have secure passwords.
In Salesforce there is not a declarative setting to prevent repetetive strings in the password however by forcing users to mix numbers, uppercase, lowercase letters and special characters you can reduce the risk caused by this.
Just having repeated strings is not necessarily insecure if the password is of a reasonable length and requires mixed characters, e.g. 12 characters with mix numbers, uppercase, lowercase and special. Example P4ssP4ssP4s!
Passwords on their own are not a particularly strong security mechanism so think of using other aspects of SF platform security.
You can also use multifactor authentication to increase security, see documentation.