Sometimes you have to show in no incertain terms that you mean business, let them (people) know that they have to get out of their comfort zone.
Sometimes you have to let them know that you are serious about it, conveying the message that they have to get out of their comfort zone. Be firm, definite, in an unwavering tone.
It´s a case of setting boundaries, so what you need is to be firm convey that you mean what you say, you can even support the said people, but they are not in charge, period. So, you are going to stick to your guns.
"Getting an attitude" may have a ring of getting resentful and hostile, it is more suited to describe tentative (sometimes immature) teenage ways.