Getting a probability of 1 suggests that the model has overfitted, and the observation is being predicted as belonging to the specific class by all trees.
You can overcome this issue by reducing the size of the trees. Few of the options that might help are,
- MinLeafSize: Set this to higher value
- MaxNumSplits: Set this to a lower value
Alternately, you may want to use a different approach entirely, by using SVMs or other classifiers.