How would I lock an axis for a curve? I don't want the curve to go in a certain direction until I unlock it. I'm frequently finding that when I created a beveled curve, what looks like everything is on the same level, is actually higher and lower on the Z-Axis.
You could try setting the curve shape to 2D in Curve > Shape until you want to adjust it in Z axis:
Note that you will still be able to rotate the curve in Object Mode, so you can use this to lock the curve in any direction.
Another option is to fix the curve afterwards:
Select all the control points (A):
0on the axis you want to lock. E.g. SZ0
This will scale everything to be on a plane with the Pivot point, so you can easily control this by using the 3D cursor: