Well, first off, Canada Customs KNOWS when you cross the border in your car.....time, date, where you crossed, everything. So if you think you're fooling them, uh uh, nope.
As for filling your suitcase up with clothes and removing the sales tags, you'd better remove the "manufactured in" tags as well. FYI: if you bring a suitcase full of clothes back with no tags whatsoever plan on paying duties and at least taxes on the estimated value of all of them. Even tho if the clothes were made in the US, (nafta) you will have to pay tax on them. You'll probably get hit with "making a false declaration" fine as well.
If you have any type of criminal conviction in Canada forget about them letting you in. I'm not sure at what level they ban you but something like DUI WILL keep you out. And yes, US Immigration will know about it.
In addition, if you buy anything at the Duty Free shop going into the US, that data is also reported back to Canada Customs. That's why they ask you for your Licence Plate Number. So when you come back in the Canada Customs agent will ask you about that 26er of Rye you bought 4 hrs ago. Now you could lie, and say you gave it to your US friend as a gift, but I bet they'll ask for that friend's name address and phone number.
BTW: Now that the Cdn dollar is back up, we're getting back into the cross border shopping thing again. About 15 yrs ago the dollar was strong and every friday night, the border was packed with Canucks going south, and sunday night with them coming back. So if you think you're fooling the customs agent with "I went to the aquarium for 8 hours" and have nothing to declare, I give you 50/50 odds that you'll get a secondary inspection. NOw a lot of it depends on their impression of you. Say a family with 2 teen girls, mom and dad in a mini van, only over the border for 8 hrs, then they KNOW you've probably done some shopping and if you say "i have nothing to declare" then start emptying out the mini van cuz they're going to look EVERYWHERE. BTW: wearing the new clothes doesn't work either, they'll check the tags.