I'm not sure I'm on the right track here, but could you disable the button for certain users, and use the onLoad event to check the user name and enable the button if the right user logs in.
Another idea might be to have a form which opens before the one with the button. Gave the user type a password in there, click a button to open the second form, and if the pasword is right enable the button.
Otherwise in the onClick event bring up a dialogue box (or a dialogue form), and have the user enter the passsword there, if it's wrong cexit the sub, otherwise go right ahead....
Here is some code that will do the trick. Can't take credit for it though, found it on another web site.
Private Sub PasswordProtect_Click()
Dim strInput As String
Dim strMsg As String
strMsg = "This form is used only by the ''Special Form'' people." & vbCrLf & vbLf & "Please key the ''Special Form'' password to allow access."
strInput = InputBox(Prompt:=strMsg, title:="Special Password")
If strInput = "YourFormName" Then 'password is correct
DoCmd.Close acForm, Me.Name
Else 'password is incorrect
MsgBox "Incorrect Password!" & vbCrLf & vbLf & "You are not allowed access to the ''Special Form''.", vbCritical, "Invalid Password"