If you think talking to children is sufficient, then you are a classic example of why it's not.
That doesn't even work with most adults.
Yes, but at the same time a resounding no!
Whether "talking to children/adults/whoever" works is dependent on a range of things:
1) Are you approaching it from a positive mindset? If you talk to people expecting them to ignore what you say, you are far more likely to find them ignoring you.
2) Are you "preaching", or trying to be informative? If the former, what you tell people will often be heard with a degree of skepticism and subsequently disregarded.
3) Have you always taken the "talking" approach, or have you historically taken the "controlling" approach and are trying something new? The former is more likely to be successful than the latter (although it does, to an extent, depend on how you are introducing the latter).
4) Are you the only one in their lives taking the "talking" approach? Particularly with kids in a family setting, consistency is key here.
5) Are you trying to "scare them into good behaviour"? If so, give up... it won't work!
6) The "Do as I say, not as I do" approach will not work. The approach of "I'm older/more knowledgeable/whatever than you, therefore I don't have to live by the same rules" will get peoples backs up. Of course, there will be areas where the activities of others (particularly kids) should be restricted more than your activities, but where this is the case it is essential to make this clear up-front, and explain the reasons why (and ensure that these reasons are understood). Otherwise, you will be seen as a hypocrite, and ignored.
The above being said, however, I would agree that, particularly with the very young, talking is unlikely to be sufficient in isolation. Back it up with monitoring (but be up-front and honest that you are doing so... silently monitoring them without their knowledge will only result in resentment), and include regular reviews of what is found in the monitoring (both "positive" and "negative"). IF you do this, however, monitor EVERYONE'S traffic in the household (otherwise, point 6 applies!). This also gives you the opportunity to set an example by way of logs of your own activities.
Above all else, make sure it is clear that they can talk to you about anything (and I mean ANYTHING). Be approachable. Be prepared for them to see some things that you might not want them to, and to subsequently have an age-appropriate (but honest) discussion about them and make it clear that you expect that this will happen to start with, so there is no point covering it up because you won't be angry unless they try to hide it (and stick to this!).
It is also worth making clear that if you are not filtering, and your child has friends over to visit, it will reflect on them very badly if they use the opportunity to bypass the parenting style of their friends' parents by showing them things they wouldn't have access to at home. Explain that it may be tempting to "show off" how you have greater trust in your kids than other parents do, but that abuse of trust will have consequences (be sure to make these consequences clear ahead of time and stick to them). It might also be worth letting the other parents know of your differing approach... If, however, you do this, make sure that you are not using an approach of "I'm a better parent", but rather just informing them of the different approach and the potential that the temptation to abuse it in front of friends may be too much for your child the first couple of visits and to be prepared to have some questions raised by their child).