The Forms collection only contains open forms, so you could loop through it,
closing everything except the current form.

Dim intLoop As Integer

 For intLoop = (Forms.Count - 1) To 0 Step -1
   If Forms(intLoop).Name <> "NameOfCurrentForm" Then
     DoCmd.Close acForm, Forms(intLoop).Name, acSaveNo
   End If
 Next intLoop

The reason for looping from the end to the front is that if you went from 0
to the end, when you deleted Forms(0), the previous Forms(1) would then
become Forms(0). When you hit the Next statement, you'd move from Forms(0)
to Forms(1), meaning that you'd never delete the form that started as form
1.

Depending on where you're calling this form, how you determine
"NameOfCurrentForm" will change.