You can follow the Microsoft TechNet guide to add VisualBasic code in Outlook rules.

You can just replace the code they give with this:

Sub AutoDeclineMeetings(oRequest As MeetingItem)

' If its not a meeting, we don't process
If oRequest.MessageClass <> "IPM.Schedule.Meeting.Request" Then
    Exit Sub
End If

' Get the appointment in the meeting
Dim oAppt As AppointmentItem
Set oAppt = oRequest.GetAssociatedAppointment(True)

' Send a decline response
Dim oResponse
    Set oResponse = oAppt.Respond(olMeetingDeclined, True)
    oResponse.Send
    
' Lastly, delete the message
oRequest.Delete
End Sub
 
Sub AutoAcceptMeetings(oRequest As MeetingItem)

' If its not a meeting, we don't process
If oRequest.MessageClass <> "IPM.Schedule.Meeting.Request" Then
    Exit Sub
End If

' Get the appointment in the meeting
Dim oAppt As AppointmentItem
Set oAppt = oRequest.GetAssociatedAppointment(True)

' Send an accept response
Dim oResponse
    Set oResponse = oAppt.Respond(olMeetingAccepted, True)
    oResponse.Send
    
' Lastly, (optionally) delete the message
'oRequest.Delete
End Sub

For anything else you may want to do with the meeting, check the Outlook Visual Basic Developer Docs.