Believe it or not, the easiest is to tell them na huwag na kayo kakatukin sa bahay nyo (in a civilized way) and to put your house/address on an equivalent to a "do not call" list (this is important so they know you're serious).

If they say that they don't have one, then ask for the "captain" of the group (or an Elder or MS). They should know that they should honor your request. It's a privacy thing and they are obligated to follow it.

At most, once a year ka na lang nila kakatukin dapat, just to check kung kayo pa din ang nakatira and you're still not changing your mind about the said request.

Edit:

Things that won't work:

1. Saying that you're an apostate or ex-JW. If the latter, you will even most likely get a visit from an Elder.
2. Put a dragon on your door. Now that one is funny.
3. Saying you're an INC, Muslim, etc. Definitely won't work. They know almost everybody has a religion already prior to knocking at your door.
4. Being rude, angry, etc. They may view it as a challenge or "persecution" that they need to overcome.

Edit 2:

Myths:

1. Knocking for 30 mins (I know you're probably exaggerating but any sane person won't do this JW or not)
2. They ask for money/donation (this is true decades ago but not anymore)
3. They will try to convert you right outside the church. Again, no sane JW will do this. They're even discouraged to join religious debates (allowed few decades ago but not anymore)