Help with soiling problem

Hi all...
My fine young man Oli has started poo pooing on the spare room bed.... why...
Recently a stray cat has taken up residence in our home.. the cat has been hanging around for best part of the year... its not a problem for us as we quite like the cat
Oli wants to play with the cat but the cat isnt bothered about oli nor does it want to play with oli so it just ignores our man and makes his way upto the saftey of our spare room...
Only problem is now Oli, who has always been an embarrassing strong scent marker has now started to wee and poo where this cat has tried to make home...
At first I blamed the stray but the first lot of wee was right on the strays pillow.. why would stray pee on its own bed?
Then the poo poo started arriving daily.... with lots of Olis incriminating orange fluff all over the place
Poor stray deserves a warm place to sleep but on the other hand I cant have Oli doing is naughty toileting in there....
Any ideas??

The only thing that I can think of is make it so the cat can get in the room but the dog cant, may be a baby gate the cat can jump over or go through the bars but hopefully the dog cant.

I would imagine the cat marked there so it smells of urine so the dog is `going` on top.
Baby gates?

Baby gate it is Oli is just pooing all over the place in there.... very strange
Thanks both for replying

Just seen some on amazon for about £15

Aww thanks...
luckily we have one down at the caravan...which we are at now.. so ill take it home with me and give it a go.... hope it works...

Hope so, good luck.