[AccessD] How do I clear a combobox? (email contains a fair amount of code)

I have a form with combobox fields for various ambulance personnel: Driver,
Assistant, Attendant1, Attendant2, and Attendant3. Each combobox has the
LimitToList property set to "No". This form is bound to tblMembers, which
includes 3 attribute flags (Yes/No fields): booMember, booMemberActive, and
booMemberEMS.

I have 3 queries on the table:

qryMembers - returns all records where booMember=True
qryMembersActive - returns all records where booMember=True And
booMemberActive=True
qryMembersEMS - returns all records where booMember=TrueAnd
booMemberActive=True And booMemberEMS=True

In the Form_Current module, I initialize the combobox fields:

Driver.RowSource = "SELECT '(Show All Members)' from qryMembersActive UNION
SELECT DISTINCT [FullName] from qryMembersActive"

Assistant.RowSource = "SELECT '(Show All Members)' from qryMembersActive
UNION SELECT DISTINCT [FullName] from qryMembersActive"

Attendant1.RowSource = "SELECT '(Show All Active)' from qryMembersEMS UNION
SELECT '(Show All Members)' from qryMembersEMS UNION SELECT DISTINCT
[FullName] from qryMembersEMS"

Attendant2.RowSource = "SELECT '(Show All Active)' from qryMembersEMS UNION
SELECT '(Show All Members)' from qryMembersEMS UNION SELECT DISTINCT
[FullName] from qryMembersEMS"

Attendant3.RowSource = "SELECT '(Show All Active)' from qryMembersEMS UNION
SELECT '(Show All Members)' from qryMembersEMS UNION SELECT DISTINCT
[FullName] from qryMembersEMS"

So far, so good - the code *works perfectly*. For the rest of this question,
I'll only refer to one - Attendant3. 

In the _BeforeUpdate module, I test for the selection of the 'Show All'
values, and if selected, I update the RowSource:

Private Sub Attendant3_BeforeUpdate(Cancel As Integer)
If Attendant3 = "(Show All Active)" Then
    MsgBox ("Please select a valid Active Member or enter a non-Member's
name (last name, first name).")
    Attendant3.RowSource = "SELECT '(Show All Members)' from
qryMembersActive UNION SELECT DISTINCT [FullName] from qryMembersActive"
    Cancel = True
ElseIf Attendant3 = "(Show All Members)" Then
    MsgBox ("Please select a valid Member or enter a non-Member's name (last
name, first name).")
    Attendant3.RowSource = "qryMembers"
    Cancel = True
End If
End Sub


My problem? I want the combobox to show a default value nothing if either of
the "Show All" options is selected, but after the Cancel=True executes,
control is returned to the combobox with the proper row source but with the
"Show All" selection showing in the combobox.  How do I blank that out?

I have tried Attendant3="" and Attendant3=Null both on the line before and
the line after the Cancel=True.

I have run out of ideas. Any suggestions?

TIA, Bob Gajewski