试了很多方法，最后发现万剑大哥的方法管用，转载一下
转自：https://www.cnblogs.com/midcn/p/3527123.html
通过测试webBrowser与IE缓存和Cookie都存放在Local Settings\Temporary Internet Files，我们可以直接调用IE API进行清除
解决方案1：
public enum ShowCommands : int
{
SW_HIDE = 0,
SW_SHOWNORMAL = 1,
SW_NORMAL = 1,
SW_SHOWMINIMIZED = 2,
SW_SHOWMAXIMIZED = 3,
SW_MAXIMIZE = 3,
SW_SHOWNOACTIVATE = 4,
SW_SHOW = 5,
SW_MINIMIZE = 6,
SW_SHOWMINNOACTIVE = 7,
SW_SHOWNA = 8,
SW_RESTORE = 9,
SW_SHOWDEFAULT = 10,
SW_FORCEMINIMIZE = 11,
SW_MAX = 11
}
[DllImport("shell32.dll")]
static extern IntPtr ShellExecute( IntPtr hwnd, string lpOperation, string lpFile, string lpParameters, string lpDirectory, ShowCommands nShowCmd);
//清除IE临时文件
ShellExecute(IntPtr.Zero, "open", "rundll32.exe", " InetCpl.cpl,ClearMyTracksByProcess 255", "", ShowCommands.SW_HIDE);
其中ClearMyTracksByProcess 可进行选择设置 ：
Temporary Internet Files （Internet临时文件）
RunDll32.exe InetCpl.cpl,ClearMyTracksByProcess 8
Cookies
RunDll32.exe InetCpl.cpl,ClearMyTracksByProcess 2
History (历史记录)
RunDll32.exe InetCpl.cpl,ClearMyTracksByProcess 1
Form. Data （表单数据）
RunDll32.exe InetCpl.cpl,ClearMyTracksByProcess 16
Passwords (密码）
RunDll32.exe InetCpl.cpl,ClearMyTracksByProcess 32
Delete All （全部删除）
RunDll32.exe InetCpl.cpl,ClearMyTracksByProcess 255
解决方案2：快速清除webBrowser Cookie
wb.Document.Cookie.Remove(0, (wb.Document.Cookie.Count - 1))