They do sit on the disk. You have to force the deselect of the DDDT project by resaving the My Grid > My Project" option, then go into the projects\www.worldcommunitrygrid.org datafolder and delete anything that looks like dddt related. If you delete and DDDT is still selected, at a point in time the software and images will be downloaded again, latest when there is again intermittent work.

But, what amount of disk space are we talking about? When you look in the client statistics you can see how much is used. Mine says 209mb in the pie with all projects active. The disk is 500 Gb and the dddt specific images take about 4mb, those that I still have. Not done DDDT for longer so cant tell what the software size is like. System requirements says 2.5mb which is the compressed size. This thread says 15mb.

You decide if it's worth to delete manually and break something. Resetting WCG is another way to achieve this though, but then you loose all of WCG including jobs in progress.

15 MB?
You make more junk trough cookies or temp from Internet in one day of surfing...
Why being bothered by 15 Mb more or less? It's like discussing about loosing more gas 'cos your car got heavier while collecting dust on tires from city streets.