Sometimes a park needs to be closed. Claiming that the park is closed is seldom enough to keep skaters away. The best way to handle temporary closures is to announce when and why the park will be closed and for how long. If the reason provides value to the skaters, such as pressure-washing, there should be little conflict.
When parks are closed over lengthy periods for reasons unclear to skaters, such as “punishment” for rules violations or maintenance concerns, the skaters are likely to overlook the closure using personal pretenses. Parks that are closed as a method of aligning behavior punishes all park patrons universally…even those that diligently obey the rules (or even encourage others to do so). It is not a patron’s responsibility to ensure that all patrons follow the rules any more than it’s the skater’s responsibility that softball players don’t have beer in the dugout. Avoid closing the park for all but the most essential reasons, such as cleaning or refurbishments.
There are two good ways of discouraging skateboarding at a skatepark: water and gravel. Skaters hate water like cats, and a wet skatepark is basically useless to all but the most foolhardy. Gravel is a serious method for preventing skating. Of course, thousands of gallons of gravel is going to make a huge mess, and may prevent you from doing the work that needs to be done, but it can fill the need for some closures.