Yes they still have the force, but they can't really do any mindgames or anything that directly affects something in the Halo universe (lighting not included). They can still use the force to move objects and throw the lightsabers though so yes, they will still have the force.

The force has much more you can do with it: Grip/throw Lightning Protective ward Maelstrom (energy shield that reflects incoming energy and objects and returns to sender) Force push Force pull Force lightning Mind trick (on weak minded) Mild telepathy Absorption/drain (absorb energy/drain life force) Blaster reflection (vader could do this with his hand for example) Force healing Analyze or scan objects/surroundings with your mind Become one with the force (turn yourself into energy and interact with the energy of the universe in that form; non reversible so u cant take physical form again) Pacify and control wild beasts and animals [spoiler] In young jedi series, the book of sith and new movie likely: darth sidious escapes death by using force transference to move from his body into a clone of his body[/spoiler] And there are a few other abilities as well