Lice and eggs that can be removed with reasonable effort are considered a chatzitzah. Lice and eggs that remain after thorough combing or other treatment are not.

In other words, you are not required to hire someone to remove all of the lice and nits. You are required to remove as much as you can with thorough combing prior to immersion. Lice and nits that elude you will not invalidate your immersion.