Variables can be unassigned using the clear function.

The clear function alone clears all variables in the workspace. All that is entered in the command window is clear. There are no inputs or outputs.
The clear variables function clears all the variables in the current workspace (see Workspace and Scoping Rules).
clear variables
The clear global function clears all global variables.
clear global
The clear functions function clears all functions except those locked.
clear functions
The clear all function clears all variables, global variables, and functions.
clear all
The clear function followed by a name of a variable or function clears the input function or variable.
clear a
clear makeSin
The clear * function clears all variables that begin with anything before the * in the input after the clear.
clear ma*
The clear function can also be used to erase specific variables passed to it:

All clear functions clear those that they are designed to clear except functions, variables, and so on, that are locked. There are functions that can be used to unlock and lock those functions and variables .