What is it about forbidden fruit?

Seamus has a kid in the neighborhood he likes to play with, let's call him Almond. When they first started playing together, I din't mind Seamus and Almond playing together. It was a nice enough set up considering Almond lives right across the street Then, I realized that Almond was a bad influence on Seamus and that Almond's mom was too busy breeding* to really care what he did, so I started trying to limit their time together. It was easy enough to do when Seamus was in preschool last year and Almond was in kindergarten. He only went for 1/2 a day while Almond went for a full day, so I just made sure he was in by the time Almond got home. This year, though, it has been harder. Seamus is in kindergarten, so they ride the same bus and get home at the same time.

I didn't expressly forbid Seamus from playing with Almond until about 4 or 5 months ago. Seamus had gone over to his house and I went to retrieve him. When I rang the doorbell, Almond's dad came to the door in his bathrobe with (obviously) nothing underneath. Something about it just didn't sit right with me and brought out my inner mama bear, so that was the last time I let Seamus and Almond play inside together. They continued to ocassionally play outside togetehr until Almond starting hitting Seamus and I forbid Seamus from playing with him. No more, I was done. There are plenty of other kids to play with.

While I've had to keep them apart a few times, Seamus has, for the most part, listened. His daddy and I are quick to correct him if we discover them together. More and more, though, I've found them playing together.

Last week, I called Seamus in for dinner and one of the kids on the playground informed me that Seamus was at Almond's house playing. I marched over there and retrieved him and immediately grounded him (not just because of this, but because he went to and into someone's house without telling me). No TV, no games, and extra chores for a week. Today is day 5 of the punishment and with Mike basically MIA because of an exercise at work, I was losing my mind listening to him whine. He had been pretty good for a few days, so I decided to end his (and my) punishment a couple of days early to recharge my sanity. Before I sent him out, I expressly reminded him that if Almond tryed to play with him and he wouldn't leave him alone, he was to come tell me because he was not supposed to play with Almond. He went outside to play on his Slip n' Slide and all was peaceful for about an hour. Then I heard Seamus and another child yelling "SOPHIE!" over and over, so I stepped out to make sure she hadn't freed herself (she hadn't). But…

Almond was here and soaking wet from playing on the Slip n' Slide with his clothes all over my yard (he still had his shorts on).

So, Seamus is now right back where he started and I have another round of punishment to look forward to. I know this isn't really working, but we're not prepared to move and I'm out of ideas. I'm sick of punishing myself while punishing him, but I have no clue what else I can do. I've told Almond to stay away from Seamus, I've told his mother, I even talked to Seamus teacher about school hours after I saw that they have recess at the same time.

So, if you have any suggestions, I'm open. How do I make my son stay away from the kid I don't want him near? 

*This isn't a slight on so-called "breeders". I have a dear friend who is a a quiverfull mom who has amazing, loved, well-behaved kids. This mom, however, breeds just to breed and ignores the kids she has and, thus, has 4 little brats and one more on the way.