A: the passive aggressive model:

So you get this second respirator and keep it handy. Preferably an old, ugly one, that is war surplus from the first or second world war, and still smells of 50 years of storage.

When they guy shows up behind you, you stop what you are doing and hand him the respirator and INSIST he put it on as long as he is near where you are working.

It is, of course, for his own safety. (otherwise someone with a power tool in their hands might injure him?)

B: the training model:

Get another pair of plastic gloves and a wood block with sandpaper on it. 320 grit or finer. Ask him to help with some light sanding.

C: the invitational:

Get an old couch or old easy chair. (This is also called a groaning chair) Offer the neighbor a place to sit and let them ramble on while you run the belt sander. Excuse yourself as you don your hearing protection and effectively block out all conversation.

d: The friendly approach:

Get a disposable camera and ask the neighbor to get a few pictures of you as you work. Later you can post them and we can see how you are doing.

e: the evangelistic approach:

Obviously the guy is envious and too macho to admit it. Give him this URL and encourage him to start on his own boat. loan or buy him a book on the subject. Give his kids books on boat building. This is of course the nastiest approach, as once he gets hooked he'll never be able to break the addiction.

f: Finally:

Give up, build him a boat, too, and you can both go paddling. Or he can make the paddles.

hope this helps

PGJ

p.s. Be happy your hobby is boat building and not home brewing. sounds like this guy might drop over to sample your brews, too.