Not sure if everyone realises this but the “free up space” feature is actually part of Windows and is controlled by attributes. As mentioned OneDrive had a similar problem so I wrote a PowerShell script to free up space in a folder. Just tried it and it works with Seadrive as well 
Here it is supplied “as is”…
# Free-OneDriveSpace.ps1: clear space used by OneDrive.
Param(
[Parameter(Mandatory)][String]$Path,
[Switch]$Show
)
# Check folder exists.
If (!(Test-Path $Path -EA SilentlyContinue)) {Write-Warning "Unable to find ""$Path"""; Exit}
# Get list of files and folders with attributes.
$Results = attrib "$Path\*.*" /s /d
# Build list of files that are not hidden and downloaded from cloud. Bit sketchy, uses output of Attrib command.
$Pattern = "^....(.)..(.).....(.)(.).*(\w:\\.*)$"
$Files = @()
ForEach ($Line in $Results) {
$Hidden = $Line -Replace $Pattern, '$1'
$Offline = $Line -Replace $Pattern, '$2'
$Pinned = $Line -Replace $Pattern, '$3'
$UnPinned = $Line -Replace $Pattern, '$4'
$File = $Line -Replace $Pattern, '$5'
If ($Hidden -ne "H") {
If ((($Offline -eq " ") -And ($UnPinned -eq " ")) -Or ($Pinned -eq "P")) {$Files += $File}
# If ($Pinned -eq "P") {$Files += $File}
}
}
If ($Files.Count -eq 0) {Write-Warning "Nothing to do"; Exit}
# Free up space.
$Count = 1
ForEach ($File In $Files) {
If ($Show) {Write-Host $File}
Attrib -p +u $File | Out-Null
$Count += 1
}
Write-Host ("Freed space for {0:N0} file(s)" -f $Files.Count)