Locking him/her in the basement ! =p ... why not?!? =D
Okok, then each advice up here is great !! And, as for me, I like to treasure moments, not people. People are free to stay or go whenever they want to...but I will always have the minutes, hours, years or centuries that we've spent 'together' . So I focus in live the instant with joy and without any feelings of losing anything... :) and that allows me to percieve my friend's presence here and now, which makes me more happy, cheerful and bright... and then we are back to the moment I will treasure... a happy one, whether this person is with me or gone.
Ok, it's a little complicated 'circle' but that's how it works for me ^^