well, put em on 1 province, FD them so they become your protectorate, and give "a punishment" for them, namely:

add_money, seleucid, -40000 (as I make Roman my protectorate, use their internal name)

repeat many times (30-40x will be good)

that way, they will be in dire ecomomic debt, and they will eventually need your protection to stay alive....

of course, they got monetary bonus, but once you deplete their money to negative millions, they will never build anything again...