SOLUTION
If poor or non-performance doesn't seem to matter to the poor performer, then you need to make it matter by rewards for good performance or punishments for bad.
Solution Explanation
Consequences are of two types: 1) negative and 2) positive. Negative consequences are those which threaten some kind of punishment for poor performance. Positive consequences offer some kind of reward or incentive for those whose performance is up to expectations.
Positive consequences have been shown through research to be more effective in increasing good performance than have negative consequences in reducing bad performance. Rewarding good performance increases the likelihood of that performance being repeated, while negative consequences for poor performance may have unpredictable outcomes such as driving the behavior underground where you cannot see it.
The first strategy should be to offer more benefits or rewards for improved performance (praise, recognition, incentives). If that should fail, then it may be necessary to warn the poor performer that continued poor performance will have negative consequences. Be sure to follow through on your warning if performance continues at substandard levels.