Just as practicing helps to establish and strengthen associations, a failure to practice helps associations to dis-associate, or lose strength. Think of associations like you would hiking trails through a forest. The more those trails are used, the clearer they are, and the easier they are to navigate from begining to end without getting lost. As trails are less used, they become overgrown. It becomes increasingly likely that you will get lost trying to follow them.
You can begin to extinguish a problem behavior by not engaging in it, and by avoiding all stimuli that would prompt you to engage in that behavior. Extinction of behavior in this manner takes a long time, and can be quite complicated, but it is possible. Stopping smoking makes for a good example of extinction. If you literally stop yourself from smoking, after a long while, you will stop having the urge to smoke, or at least that urge will lessen considerably. However, it will be extremely difficult for you to pull this off, because cigarettes contain an addictive drug which will cause you to go into withdrawal, and because your smoking habit is deeply ingrained into most every aspect of your life. Many habits you have, many objects you own or use, will remind you of the pleasures of smoking and reinforce you to light up again in a positive reinforcement sort of way. You may experience aversive cravings as well, which also reinforce you to light up through a process of negative punishment (e.g., you will feel better if you light up). If you can hold out against all this pressure to smoke, your urge to smoke will ultimately lessen in intensity. You can dramatically improve your odds by getting rid of (avoiding) as many objects associated with your smoking habit as possible before you begin, and by getting medical attention for your addiction and withdrawal symptoms.