We can shave off even more time if we eliminate the checks when adding
items to the dictionary because the dictionary won't allow dupes...

Function StripDupes(Optional AllowDupes As Boolean = True) As Boolean
' Compares colA to colB and removes colA matches found in colB.
' Args In: AllowDupes: True by default. Keeps duplicate values
' found in colA that are not found in colB. If False,
' duplicate values in colA not found in colB are removed.
'
' Returns: True if matches found and no error occurs;
' False if matches not found --OR-- error occurs.
'
' Sources: Ron Rosenfeld, Jim Cone, Garry Sansom

Dim i&, j&, lRows1&, lRows2&, lMatchesFound& 'as long
Dim vRngA, vRngB, vRngOut() 'as variant
Dim dRngB As New Dictionary

On Error GoTo ErrExit

lRows1 = Cells(Rows.Count, "A").End(xlUp).Row
lRows2 = Cells(Rows.Count, "B").End(xlUp).Row
vRngA = Range("A1:A" & lRows1): vRngB = Range("B1:B" & lRows2)

On Error Resume Next
For j = LBound(vRngB) To UBound(vRngB)
dRngB.Add Key:=vRngB(j, 1), Item:=vRngB(j, 1)
Next 'j
On Error GoTo 0

If AllowDupes Then '//fastest
For i = LBound(vRngA) To UBound(vRngA)
If dRngB.Exists(Key:=vRngA(i, 1)) Then _
vRngA(i, 1) = "": lMatchesFound = lMatchesFound + 1
Next 'i

j = 0: ReDim vRngOut(UBound(vRngA) - lMatchesFound, 0)
For i = LBound(vRngA) To UBound(vRngA)
If Not vRngA(i, 1) = "" Then _
vRngOut(j, 0) = vRngA(i, 1): j = j + 1
Next 'i

Else '//slowest
Dim dRngA As New Dictionary
On Error Resume Next
For i = LBound(vRngA) To UBound(vRngA)
If Not dRngB.Exists(vRngA(i, 1)) Then _
dRngA.Add Key:=vRngA(i, 1), Item:=vRngA(i, 1)
Next 'i
On Error GoTo 0

Dim v As Variant
j = 0: ReDim vRngOut(dRngA.Count, 0)
For Each v In dRngA
vRngOut(j, 0) = dRngA(v): j = j + 1
Next 'v
End If 'AllowDupes

Range("A1:A" & lRows1).ClearContents
Range("A1").Resize(UBound(vRngOut), 1) = vRngOut

ErrExit:
StripDupes = (Err = 0)
End Function 'StripDupes()

--
Garry

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