How to secure opening to prevent access?
I'd put a single 4x4 fence post at the alley end (or the other end, depending on your aesthetic preference), set flush on the face with the building wall(s) or recessed slightly and centered. This essentially leaves you with 5" gaps on either side--too small for most people to pass. Paint it to coordinate. 

Plan view

|             |
|    gap      |
|             |
|    ____     |    garage
|    |   |    |
|    |___|    |___________________________________
|             
|      ^ --------- post
|      
|


I'd then lay heavy landscape fabric (not plastic) down the entire length of the gap and place 2" landscape rock on top. You should have very few weeds and it'll look finished.