Somehow when you try to thicken the surface body in order to convert it become a solid model with a defined thickness, you may face this kind of situation where due to the geometry of model, some area of the solid model will have excessive material which is unwanted as shown in the figure above.
In this situation, you need not to use sketch and extrude cut in order to remove the unwanted geometry. This will pretty take time to sketch a close contour to fully enclose the unwanted volume and you may not get the nice result as you want!
Try on the Delete Face command first! This is the fastest way to help you repair the geometry of the model. Select the face belong to the unwanted geometry and under the option for Delete Face command, choose " Delete and patch", means, the software will remove the selected face and patch back the model with a more predictable geometry. (If you select option"Delete" only, it will only remove the selected face and you solid model will turned into surface model). Refer to the figure below, this is the outcome of the delete face command! Simple and nice?
Sometimes, Delete Face might not able to solve your problem, it's all depends on the complexity of model geometry just like the situation as shown in the figure below:
In this case, you can try the 2nd alternative which is Replace Face command! Let see how simple it is! First, do a offset surface for the face that you feel is suitable to be used to patch back the unnecessay geometry, make sure the offseted face is large enough to cover the target face to be patched! Or else, you will need to extend it!