For starters, you want to test for -1 and 0 not "-1" and "0" (drop the quotes). Secondly, I would simply use the after update event of the checkbox and use an Else clause to handle the False. Finally, the task at hand can be easy or tricky, depending on what you're adding. Given your example, it would be fairly easy. This should delete "MAA" from the textbox:
Code:
If (Me.Checkbox1) Then
Me.Text187 = "MAA" & vbCrLf & strcomment & Me.Text187
Else
Me.Text187 = Replace(Me.Text187, "MAA", "")
End If
That said, it would also take out any other instances of "MAA" that might appear in the textbox, so it could get problematic. I would probably not try to build this string until the user was done making their selections. That way you don't have to worry about undoing anything.