use a splitline to split the face so that you can apply the restraint to only one part of it
I can't see where your planes are but you can sketch on one or more of them and choose the surface you want to split (way to the right/bottom) in the Features tab. You might have to split it in two directions depending on the exact shape/area you want to end up with.
When you do fix the new surface you have the choice of restricting 3DoF, or any combination of radial, axial or rotational.
Thank you Jared and James, With your advice I managed to split the surface. Now I just need, as James said, to define the exact shape of the surface on which I´m gonna apply the restraints.