How do you handle clients that are too affectionate?

Hi,

I have just started working in dd grouphomes (i'm not a nurse yet), and i have wanted to do this kind of work since i was in high school.... but i have one little problem: in each of the grouphomes where I work, there is one client that is.... just a little too affectionate. In the first home, its a 12-year old boy. He does whatever it takes to get my attention, and if he doesnt get it (when i am busy helping someone else), he'll do drastic things like bang his head against the wall etc.... whenever i am in the room, he hangs on me, when i sit on the couch, he practically sits on my lap. He can barely talk, and i am not sure how much he understands. The other staff say he doesnt have an MR diagnosis, so he understands alot, but at the same time, the books and games and videos he has are all at a 3-5 yr old level. so i just dont know....

At the other home, its a 28-yr old TBI client who is mentally at the level of a 4-5 year old, but he also will sit really close to me, tell me that i am cute, try to kiss me etc.... i am new at this and it makes me a little uncomfortable, sometimes to the point where i avoid working with him and work with the other residents instead (we always have two staff on).
Since i just started this job and already have two "overly affectionate" clients, it makes me guess that this is a rather common problem- so maybe many of you have dealt with it in the past and can give me some advise. I do really like this work, but i just dont know how to enforce bounderies, especially when i am sometimes not sure about the level of understanding of a particular client....

any advise would be greatly apprecciated!