Solution: http://stackoverflow.com/questions/16346227/export-data-to-excel-file-with-asp-net-mvc-4-c-sharp-is-rendering-into-view

ExportToExcelService.cs

public class ExportToExcelService
{
    public void ToExcel(HttpResponseBase Response, object clientsList)
    {
        var grid = new System.Web.UI.WebControls.GridView();
        grid.DataSource = clientsList;
        grid.DataBind();
        Response.ClearContent();
        Response.AddHeader("content-disposition", "attachment; filename=FileName.xls");
        Response.ContentType = "application/excel";
        StringWriter sw = new StringWriter();
        HtmlTextWriter htw = new HtmlTextWriter(sw);

        grid.RenderControl(htw);
        Response.Write(sw.ToString());
        Response.End();
    }
}

index.cshtml

    <h3>Exporta dados para excell</h3>
    @Html.ActionLink("Export to Excel", "Excel")

WhateverController.cs

// GET: ClientResponse
[Authorize(Roles = "Admin")]
public void Excel()
{
    var customClaim = Request.GetOwinContext().Authentication.User.Claims.First(c => c.Type == ClaimTypes.UserData).Value;
    var userData = JsonConvert.DeserializeObject<LoginSerializeModel>(customClaim);
    var authorization = userData.TokenType + " " + userData.AccessToken;

    var estabsUsers = _ctx.Establishments.ToList();
    var clients = _ctx.Users.ToList();

    var clientsViewModel = new List<ClientViewModel>();
    foreach (var c in clients)
    {
        var list = _ctx.Payments.Where(x => x.Owner_Id == c.Id);
        var listMonth = _ctx.Payments.Where(x => x.Owner_Id == c.Id && x.Date.Month == DateTime.Now.Month);
        clientsViewModel.Add(new ClientViewModel(
            c.Name,
            c.Email,
            c.PhoneNumber,
            list.Any() ? list.Sum(x => x.Value) : 0,
            listMonth.Any() ? listMonth.Sum(x => x.Value) : 0
            ));
    }

    ExportToExcelService exportToExcelService = new ExportToExcelService();
    exportToExcelService.ToExcel(Response, clientsViewModel);
}