Not easy, illegal, but possible
Transmitting without a license in a frequency band somebody has paid money for is illegal. Blocking is possible, but not so simple.
Typically a cellular network has a very large variance in the link margin. This means that at some places you need very little extra noise to block, and at other places you'll need to add a lot of noise.
What you may attempt to do is to take advantage of the fact that you are much closer to the cell-phone than the base-station transmitting to it, and you also know the direction to it.
Try transmitting amplified noise. Use a directional antenna with very narrow horizontal and vertical beam-width (NOT an omni) and point it at the cell phone you want to interfere.
This way you will interfere less with other phones, and also direct all your power where you want it to go.
If you transmit enough power (when enough depends on the local link margin), you'll cause the cell phone to lose the base-station.