You've had some great advice here.
Tali did this a few weeks ago before the upset tum. His anal glands needed expressing and as I told you at the time, the behaviour stopped immediately afterwards.
I use 3 things when trying to removing things from him. The first is 'Ahh no safe' which he allows me to go in and extract whatever is in his jaws. I use it very sparingly and only in emergencies.
The second is 'show Melli', which is usually post or a letter he's swiped from somewhere and he will slowly give it up.
Occasionally, although he is not a resource guarder, he will have a 'special' something he doesn't want to give up and runs away with it. Under this situation I gently apptoach and praise him and say 'ooh wot you got', ask if I can have it and when he tugs at it I say 'do you want a treat?'. The jaws let go immediately.
Obviously, you need to teach 'treat' and treat him too.
If he had an upset tum, I would be very inclined to have a quick trip to the vets to check he's ok aparts from that. Tali is pushing boundaries at the moment. We had a bad moment yesterday when he stole his 'free cake', but although I freaked he allowed me in to removed the cake without any issues. Jam and cream sponge and icing would not have done him any good at all.