Import-Excel returns a series of PSCustomObjects. The properties of the objects are the names from the column headers in the worksheet.
Give this a try:
$User = "max.muster@microsoft.com"
$AdminSite = ****************************
Connect-SPOService -URL $AdminSite -creds
Import-Excel C:\temp\testim.xlsx -startcolumn 4 |
ForEach-Object{
Remove-SPOUser -Site $_.testi -LoginName $User
}