If the restoration possesses open margins, you may have to use a finishing bur to remove residual cement at the margin without creating a defect or void. Please keep in mind that cements are not intended to be used to fill significant marginal gaps. If the margins are closed, then try gently pushing the cement towards the gingiva with a scaler to break it away from the restoration. Interproximal cement can be removed with a similar technique, pushing the cement towards the gingiva, away from the margin. Remember to dislodge any loosened fragments.