The question of how to close a Tac Force knife can be addressed in many ways. The most logical and easiest is to remove the blade and hand on the butt of the knife to one side of your assailant. If they are still alive, you may want to punch them in the face or stab them with the blade to disable them for a few moments. Some people suggest that you push them against the wall or against an object such as a trashcan to disable them for a few seconds until more help can arrive.
In order to answer the question how to close a tac force knife quickly, you have to be quick in moving the blade towards your assailant. If you are fighting back with a friend or two, this may not be possible. If you are on your own however, you must get the blade out of their reach as quickly as possible. This means grabbing them by the scruff of their neck, pulling back the blade, and delivering a strike to the throat/head/nose/ear/eye area. If you have a round blade, this should be easier than using a square one due to the rounded tip of the blade.
When fighting back, try to keep your arms and hands unblocked. Try pushing the blade away from your face and toward your throat. The only exceptions to this rule are if you are defending yourself against a knife throw, which does not allow for a push as described above, or if you are trying to flip them the other way during a grappling match. Regardless, it is important to remember that you have some leeway with the blade. Make sure that you respect the personal space that they are occupying, and do not let them have free reign on your body.