I'm using one of my school's copies of Solidworks to do a personal project. I am nearly done, all that is left is to cut everything below the spline off. Unfortunately, I don't have any real idea how to do that. I cannot extrude cut it like I did the rest of this and all other cutting options are greyed out. I'm sure I'm doing something very wrong but I don't know enough about Solidworks to know what to search.

If anyone could help me figure it out or show me how to do it, I would be very grateful.

Thanks in advance!