Do you have control over the WMS? If you do then it is probably better to create a new layer for the labels of which you can toggle the visibility on the client side. Otherwise I don't think it is possible.
I agree with FC Basson. You need to add a separate layer with labels turned on (symbology with No Color). Toggle the display of this layer to control the visibility of labels.
You can turn the visibility of service and service sublayers on and off using the check boxes in the table of contents. You cannot toggle the visibility of WMS sublayer labels.