C# has this option in InternetExplorerOptions.cs:
public bool EnsureCleanSession
{
get { return this.EnsureCleanSession; }
set { this.EnsureCleanSession = value; }
}
So what you need is something like
var options = new InternetExplorerOptions();
options.EnsureCleanSession = true;
// ...
IWebDriver driver = new InternetExplorerDriver(options);
If you are using IWebDriver driver = new RemoteWebDriver(...)
as you said in the comments, then you can
var options = new InternetExplorerOptions();
options.EnsureCleanSession = true;
DesiredCapabilities cap = (DesiredCapabilities)options.ToCapabilities();
cap.SetCapability(CapabilityType.BrowserName, DesiredCapabilities.InternetExplorer());
// continue adding other capabilities
IWebDriver driver = new RemoteWebDriver(cap)