@erhanevgin The client chooses which AP to connect to, the only way the AP can influence it is with client load balancing, have you got this turned on? If not, does the client get a better signal from the one on the other floor? If so, you could try turning down the max signal strength of the ones you don't want the clients connecting to.
Does this cause an operational issue, we often have clients connecting to APs though floors and ceilings and it doesn't cause a problem, so we don't worry.
Finally, you could create a separate SSID for each floor and use tags to make it only advertise from the specific APs on that floor.
