You need to do the following:
- extract the u- and/or v-isocurves that you want to use for trimming (or any other surface curve, does not need to be iso-curves)
- convert the surface to a Brep
- use BrepFace.Split(curves) to split the one face of the Brep
- choose which part to keep of the resulting Brep.
https://developer.rhino3d.com/api/RhinoCommon/html/M_Rhino_Geometry_Surface_IsoCurve.htm
https://developer.rhino3d.com/api/RhinoCommon/html/M_Rhino_Geometry_BrepFace_Split.htm