In a single-hand grab, four fingers are on one side, and the thumb is alone on the other. If you put the force of your whole arm and body against that lone thumb, it will give way, even if the attacker's grip is strong.
To ensure you use the force of your whole body, practice the following Kungfu / Tai Chi movements:
1. Wave Hands Like Clouds
Rotate your arms from the elbows in relaxed circles. The hands swing down to the outside, in towards your waist, then swing up the centerline of your body to your face. When you are looking directly into your palm, rotate the hand out and away to the side, and let it drop down to the outside to repeat the circle.
Application: Your attacker is facing you and grabs your wrist with his nearest hand.
If your attacker grabs your wrist with his opposite hand (his left
hand grabs your right wrist) his thumb will be on the inside. Use Wave
Hands Like Clouds to brush off his grip. In a relaxed, but firm,
fashion swing your hand in across your body, then up to your face.
Rotate the palm outwards, levering against his thumb, and wave the hand to
the outside, breaking his grip.
Important: the first movements must be very smooth and relaxed. Don't jerk your hand up to your face; swing it up lightly.
Only the final movement in the last two pictures above is firm and sharp.
2. White Crane Beats Wings ( Swat the Bug! )
This is just the opposite of Wave Hands. The hands swing up to the outside, and back towards your face with the palms turned out. When your hand reaches your face, rotate the palm in toward your face, and let the hand continue to swing down the body centerline to your waist.
Application: Your attacker is facing you and grabs your wrist across with his far hand.
If your attacker cross grabs your wrist with his same hand (his right hand grabs your right wrist) his thumb is on the outside. Use White Crane Beats Wings to brush his grip away. In a relaxed, but firm, fashion swing your hand out away from your body, then up to your face. Rotate the palm inwards, cutting in against his thumb, and swing your hand down your body centerline, breaking his grip.
These moves, combined with a loud, firm, verbal indication of your annoyance ( "NO!"), can prevent situations from escalating.