@Vaikaumaverick
I would suggest you use command below to check whether could find correct users first:
Get-User | where{$_.CountryOrRegion -eq "United States"}
Then, you can use this command below to remove photo from Exchange online mailboxes, it will sync to other Office 365 places:
Get-User | where{$_.CountryOrRegion -eq "United States"} | Remove-UserPhoto
To prevent user from modifying photo, you need to add limitation from many places, such as, from Exchange online you need to modify OWA policy to prevent user from modify photo in OWA(You could create a new OWA policy to apply on the mailbox that you want to limit). About other limitation such as Teams, you could have a look about this articles: How to prevent users from changing profile photos in Microsoft 365
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