How to modify a unified auditing policy to make exceptions based on login information

The audit policy had been created as follows:

create audit policy all_select_policy actions select;

Here is how you can alter an existing policy so that the policy will make an exception for session created by user "DBAADMIN"

alter audit policy ALL_SELECT_POLICY 
condition 'SYS_CONTEXT (''USERENV'',''CURRENT_USER'') NOT IN (''DBAADMIN'')' 
evaluate per Session;

Documentation can be found here
The oracle-supplied policy ORA_LOGIN_FAILURES automatically audits all failed login attempts. You can alter it to exclude certain uninteresting connections, such as for example DBSNMP, like this:

alter audit policy ORA_LOGON_FAILURES
condition 'SYS_CONTEXT (''USERENV'',''CURRENT_USER'') NOT IN (''DBSNMP'')'
evaluate per session;

For more information about the SYS_CONTEXT function, check the official 12.2 documentation.