Can a person with harmful thoughts still meditate? If a person has enemies and wants to take revenge, then can he meditate? Will those thoughts stop the person from meditating even if he doesn't think those things while meditating?
If a person has enemy's and wants to take vengance, than can he do meditation?


Of course he can. Insight meditation is the only way to permanently uproot the causes for those thoughts, i.e. the root defilement of hatred and namely Ignorance (avijja).


  Will such thoughts never let the person do meditation even if he don't thinks such things while meditating?


All formations are impermanent so its not possible for something to "never let" anyone do something. That's an extreme view.

It might be more challenging to do meditation while the hindrances are present. But it is necessary. One can only purify the mind by dealing with the hindrances. It just means that there is work to be done.

Loving-kindness meditation is a great way of over coming thoughts of hatred, vengeance and the like. See my answer here for a detailed description on how to practice Metta meditation.