At first you start with your "house" i say that in the help box, then the increase userbase is the only task the marketing staff do, why would you want to unclick it? And the other comment i'm sorry for that crash i'll see that, you can save the game too
The reason to unclick it is that in the early game, the cost of users exceeding the server capacity exceeds the revenue those users bring in. One of my games I went bankrupt because I had something like 60,000 users on a 50,000 capacity with users only bringing in about 0.07 each. This was enough to take my green profit and turn it into a red negative.
But the increase userbase button only works if you have marketing staff, so if you don't want to increase an extra of users apart from the "mouth to mouth" sharing just don't hire marketing staff