He could bat Ianto's hand away, forbid him that extra stimulation, but this isn't so much an intense D/s scene as just a boss/secretary one and he's fine with Ianto fisting his cock. It leaves his own hands free to brace firmly against the desk and fuck him hard, and at exactly the right angle.
"Like this?" he teases breathlessly, his cock raking over Ianto's sweet spot on every thrust now. "Gonna come for me, Ianto? Gonna spill yourself all over my beautiful desk?"