This paper introduces a novel method to causally satisfy hard position and velocity constraints in wave energy systems. The constraint mechanism is simple to implement, computationally efficient, and does not require complex tuning or optimisation techniques. The proposed strategy handles system constraints by modulating a velocity reference with a Gaussian-like envelope function that depends on both position and velocity, which results in nonlinear closed-loop dynamics. In this context, this paper focuses on the stability of the constrained closed-loop dynamics, and it is proven that, for a set of initial conditions within the constraint region, the system trajectories remain within the prescribed limits. Finally, insilico evaluations demonstrate that the Gaussian-like function effectively guarantees compliance with system constraints and is broadly applicable to wave energy systems.