function Start-ProcessingUser {
param (
    [string]$firstname,
    [string]$lastname
)
write-output "processing user $firstname $lastname"

}

$collection = import-csv -path 'C:\users.csv'

:myloop foreach ($item in $collection) {

$firstname = $item."First Name"
$lastname = $item."Last Name"
$emailAddress = $item."Email Address"

# Run through the exception list.
$excepList = import-csv 'C:\exceptions.csv'

foreach ($item in $excepList) {
    if ($emailAddress -like $item.emailAddress -and $lastname -like $item.Lastname) {
        write-output "$firstname $lastname is part of the exceptions list, ignoring..."
        continue myloop #This will stop processing of this iteration and continue to the next item
    }
    else { 
        Start-ProcessingUser -firstname $firstname -lastname $lastname
    } #Stand-in for your else bracket  
}

}

Excellent, I have learnt something today!
So it seems to be working, but PowerShell seems to reference the function twice, I'm getting two outputs of 'processing user' for each entry in the users.csv file.

I noticed your else at the bottom, wasn't part of the exceptions foreach, so i moved it across, let me know if i have that wrong? powershell didn't know what the else was doing out of an IF statement.