What is the ruling of Islam in the case of someone assuming the role of Imam though he is not qualified and the Muslims do not want him to lead the prayer (they hate his Imamaha)?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
The Prophet in a good Hadeeth counted the person who forces himself to be Imam although people do not like him among those whose prayers do not extend beyond their ears. [Al-Iraqi and Al-Albani].
So, no Muslim having knowledge of this Hadeeth should place himself as Imam of some people who do not want him.
Moreover, two points are of major importance:
First, failure to find the Imam acceptable should be based upon his failure to comply with the religion.
Second, his unacceptance should also be the opinion of the majority of the people who pray with him, not just a few.
Allah knows best.