The Drawing Setting dialog from the Geometry menu allows you to modify options associated with snapping to objects in the model, the use of dynamic line constraints and other options associated with graphically interacting with the model.

The Lock Geometry option in this dialog provides a means for stopping the geometry and topology of the frame from being modified.  This includes disabling all drawing, dragging and generation of structural members.

This dialog also provides some options for controlling how far the cursor will move to snap to an object in the model.  The Joint snap tolerance specifies how close the cursor has to be to a joint to snap to the joint. Similarly the Member snap tolerance specifies how close the cursor has to me to a member (or point on a member) to snap to the member.  In general the member snap tolerance should be equal to or smaller than the joint snap tolerance.  By default these values are quite small but for most models tolerances of 20 pixels or greater can be used.