I've had the problem of gnomes being stuck on the edge of the lot. I fixed it by getting a Sim to kick them back into the lot. It took me a while to work it out but it never fails when I do it now.
I find that putting things on the edge of the lot so a bit of them is hanging over the edge is a great way to delete things you can't delete.
For example I had a sculpture a Sim started on the sculpting thing. I don't know which Sim started it.
None of them could continue working on it.
I couldn't delete it.
I used move objects to move it but no more than that
I deleted the sculpting station and it deleted successfully. The sculpture stayed.
I eventually moved it to the edge of the lot.
Still couldn't delete it and it stayed there a few sim weeks and I was frustrated.
Then I moved the lot to the clipboard and deleted the lot and replaced the now empty lot with the one on the clipboard and found the sculpture was gone.
So now I have found this is a wonderful method of deleting things that are in use and can't be deleted. I move them to the edge of the lot and put the lot in the clipboard and delete the lot and move the clipboard version back. It has worked every time so far.
NOTE: Save the game before trying this. And check that relationships are intact with other families in town before saving the fixed game.
Edit to fix some typing I did on my phone at about 2am. Plus add a bit of extra info.
http://s5.postimg.cc/qpb5j4t8n/Gnomes_Vamp_And_Mysterious_Chi.jpg
I didn't get a picture of the vampire gnome in the cot with the baby. And I couldn't put it back into the cot later to get a picture. So it seems it can only be in the cot if it was a teddy bear that got turned into a vampire gnome while in the cot.