What should I do regarding the lease agreement after I learned that the tenant's actual activity is different from what he previously stated? What is the ruling on the rent I have collected since I became aware of this? Am I obligated to evict them and terminate the contract immediately, taking into account the improvements they have made and the possibility of them leaving the apartment in a bad condition?

If the broadcast channels are impermissible, then it is not permissible to assist in their broadcasting, and renting the apartment for this purpose falls under the category of assisting in sin. If the questioner was unaware of the forbidden activity, there is no blame on her for the current rental until the contract expires if she is unable to terminate it. She must call its occupants to goodness, enjoin what is right, and forbid what is wrong to the best of her ability, and she should not renew the rental contract for them.