Something like that should do the trick:
gciGet-ChildItem master:\content -rec |
where { $_.Locking.IsLocked() } |
whereWhere-Object { $_.Locking.GetOwner() -eq "sitecore\USERNAME" } |
% { $_.Locking.Unlock() }
You just take all the child items of the sitecore\content
item, check if your chosen user is the owner of it and unlock the item.