No need to feel guilty about wasting thier time by rejecting thier invitation, just get them to reject you, its so easy, you could pretend that you really really want to get Baptised, and they will say no, go away.
There are plenty of questions you can put to them that you will find here on this forum.
Here are some points I would raise if they ever bothered to ask me to come back.
"In John Chaper 6, Jesus specifically states that if you want everlasting life, you have to eat his flesh and drink his blood, you will not get life if you dont, so I will obey Jesus by partaking at the Memorial each year, is that OK?"
"1914 was arrived at by Russells measurements of a Satanic pyramid, is it right we should belive what the Devil says and therefore call Jesus a liar when he says, "Truly I say to you, this Generation will by no means pass away""
" If I see a JW commit a gross sin, is it true that I would be sinning if I failed to report it?, in which case doesnt that mean I should share my knowledge of the Watchtower society riding on the back of the Beast when it joined the United Nations?"
"Why do you claim the ressurection occurs at the beginning of the 1000 years whereas it states here in Revelation that it happens at the end?"
They will soon realise you are not gullible enough for them.