.NET (Core and Framework)

Shared Iterator Function GroupAccounts(ByVal lines As IEnumerable(Of String)) As IEnumerable(Of IList(Of String))
    Dim currentAccount As List(Of String) = Nothing
    For Each line As String In lines
        If String.IsNullOrWhiteSpace(line) Then Continue For
        
        If String.Equals(line, "[account]", StringComparison.OrdinalIgnoreCase) Then
            If currentAccount IsNot Nothing Then Yield currentAccount
            currentAccount = New List(Of String)()
        End If
        
        If currentAccount IsNot Nothing Then
            currentAccount.Add(line)
        End If
    Next
    
    If currentAccount IsNot Nothing Then
        Yield currentAccount
    End If
End Function

Shared Iterator Function UngroupAccounts(ByVal groupedAccounts As IEnumerable(Of IEnumerable(Of String))) As IEnumerable(Of String)
    Dim started As Boolean = False
    For Each account As IEnumerable(Of String) In groupedAccounts
        If started Then
            Yield String.Empty
            Yield String.Empty
        End If
        
        For Each line As String In account
            Yield line
        Next
        
        started = True
    Next
End Function

Shared Function RemoveUser(ByVal groupedAccounts As IList(Of IList(Of String)), ByVal userToRemove As String) As Boolean
    Dim found As Boolean = False
    For index As Integer = groupedAccounts.Count - 1 To 0 Step -1
        If groupedAccounts(index).Any(Function (line) String.Equals(line, "user = " & userToRemove, StringComparison.OrdinalIgnoreCase)) Then
            groupedAccounts.RemoveAt(index)
            Found = true
        End If
    Next
    
    Return found
End Function

Dim sourceLines As IEnumerable(Of String) = IO.File.ReadLines(textpath)
Dim groupedAccounts As IList(Of IList(Of String)) = GroupAccounts(sourceLines).ToList()
If RemoveUser(groupedAccounts, txtuser.Text.Trim()) Then
    Dim resultLines As IEnumerable(Of String) = UngroupAccounts(groupedAccounts)
    IO.File.WriteAllLines(textpath, resultLines)
End If

user<space>=<space><trimmed username to delete>

user = usertest

Shared Function RemoveUser(ByVal groupedAccounts As IList(Of IList(Of String)), ByVal userToRemove As String) As Boolean
    Dim found As Boolean = False
    Dim pattern As New Text.RegularExpressions.RegEx(
        "^\s*user\s*=\s*" & Text.RegularExpressions.Regex.Escape(userToRemove) & "\s*$", 
        Text.RegularExpressions.RegexOptions.IgnoreCase)
    
    For index As Integer = groupedAccounts.Count - 1 To 0 Step -1
        If groupedAccounts(index).Any(Function (line) pattern.IsMatch(line)) Then
            groupedAccounts.RemoveAt(index)
            Found = true
        End If
    Next
    
    Return found
End Function

Shared Iterator Function UngroupAccounts(ByVal groupedAccounts As IEnumerable(Of IList(Of String))) As IEnumerable(Of String)

txt = txt.Replace("男の子", "Boy")

string boySearchString = System.Text.Encoding.Convert(
  System.Text.UnicodeEncoding.Unicode, 
  System.Text.Encoding.GetEncoding(932), 
  System.Text.Encoding.Unicode.GetBytes("Boy"))

txt = txt.Replace("男の子", boySearchString)

Bastard Programmer from Hell
If you can't read my code, try converting it here[^]