hmm. Its difficult, since the parts you want to force are not extremities. Its natural to think arms, legs, head, and trunk. These bits stick out.
You could get rid of the trunk by saying I dont want you to think of any of the rude bits", gesturing over your upper and lower body. This then leaves them with the head, arms, and legs.
Maybe, just maybe you could plant eyes into their head by using the word Visualise. There being two hint words in one, Visual and eyes.