disable error.log for specific directory

For the errorlog there is no built in way to achieve this. You could use log piping to do this by piping the log messages to something that will ignore messages containing some specified string.

Define a pipe to handle the error log

ErrorLog "|/usr/local/bin/httpd_errors >>/var/log/apache2/error.log" 

The contents of the httpd_errors script can be as simple as

#!/usr/local/bin/perl

$|=1;

while (<STDIN>)
    {
     $ErrorMessage=$_;
     if ( $ErrorMssage !~ 'phpmyadmin' )
        {
         print $ErrorMessage;
        }
    }

This waits for messages to arrive on stdin and only prints them if they do not contain the string phpmyadmin.

If you want to ignore multiple error messages then

#!/usr/local/bin/perl
#unbuffer stdout
$|=1;
#add the strings to match below separated by |
$filter = 'phpmyadmin|setup.php';
while (<STDIN>)
    {
     $ErrorMessage=$_;
     if ( $ErrorMessage !~ m/($filter)/  )
        {
         print $ErrorMessage;
        }
    }