How to dynamically enable and disable pickmap constrain

Imagine this scenario : You have a picklist that, under certain conditions, must have a constrain in the pickmap list, so what should I do to disable it when is not necessary?
Easy, read tools online help.

Pick Map UpdOnlyIfNull

Marks a copy pick map (read Pick Map) to perform only the copy operation if the field specified in the pick map updonlyifnull is null. Constrain pick maps are also copy pick maps. In cases where the constraint is active, this results in no operation (no-op). In cases where the UpdOnlyIfNull causes the constraint to be ignored, the copy operation works. To avoid the copy operation use a calculated field as the field for the constrain pick map.