Thank you all for your help.
Yes i did try everything you have suggested before but it didn't fix my problem.
Finally, i added a script task executing the following script to close and dispose any EXCEL.EXE running while i'm executing my package :
Public Sub Main()
Dim procList() As Process = Process.GetProcesses()
Dim k As Integer
For k = 0 To procList.GetUpperBound(0) Step k + 1
If procList(k).ProcessName = "EXCEL" Then
Dts.TaskResult = ScriptResults.Success
and it works fine now 🙂