I had a similar problem and used the following code:
System.Diagnostics.Process[] procs = System.Diagnostics.Process.GetProcesses();
for (int i = 0; i < procs.Length; i++)
{
if(procs[i].ProcessName == "EXCEL")
{
procs[i].Kill();
}
}
This worked pretty well, but I would really think about working with Office on a server.