By chance is there a short cut to Cancel all sends in a session ?

Not all at once AFAIK but you can use quick grouping to make it quicker. Make sure no tracks are selected, hold down ctrl and then click one of the send on/off buttons.

That will invert the position of all the send power buttons for that send position i.e the first send on each track if you click the first one. the second send on each track if you click the second one etc

It gets a little more complicated if some are on and some off as it inverts the current position state but you can use track selection to get round that. Not ideal but quicker than doing them all one at a time.