HI Martijn,
Try Fill and Smooth. Maybe also use Inflate a bit. Or try the Clay Brush, together with Smooth.
Sometimes these bits can be really stubborn and come back, as soon as you try to Smooth. In that case, you can also go into Edit Mode and Delete the obnoxious Vertex/Vertices and then just Fill the hole and switch back to Sculpt Mode. This is, if you use Dynamic Topology. This method won’t work if you use Multires Sculpting.
