No, Alli doesn't work that way. You got the part right about Alli (orlistat, xenical) prevents fat from being absorbed from the food you eat but it does nothing for the fat already in your body, body tissue, fat tissue, bloodstream, etc. If you are trying to get clean after long term use by diet and exercise, Alli could be useful in that sense but no, no sponge soaking up THC and leaving your body.