Where can I find a mod to stop neighbors from inviting themselves inside, or at least from inviting themselves over for dinner? It bugs me when I have someone make food for the family and make a table for exactly the number of people the family has in it and then random people walk into the house and start eating our food, leaving the actual family members with no place to sit.

Oh, something to stop literally EVERYONE from showing up and kicking over the trash can would be nice too.

They walk in because they either have a lot of outgoing points or your sims are very good friends with them. You can either make sims with less outgoing points, use the simblender to take away all the BFF's BF or try locking your doors. If they are walking in and do not meet that criteria (outgoing, best friends) then you have a problem, probably with something in your download folder.

Simply use some fence around 3 sides of your bin/trash can.

There's also this: http://cyjon.net/node/308 by Cyjon for the walkbys.