Just like every other language.

You either

- Expose the entire language and then there's not much you can do to prevent something harmful.

- Expose a subset of the language perhaps through parsing for harmful patterns or by somehow making potentially harmful functions unreachable.

- Expose a more restricted language (scripting like c/lua) that you control and know its full capabilities.

The only difference I can think of is that it's easier to write flexible and dynamically modifiable software in lisp than in most mainstream languages.