Q: how can i block another wireless network?

What do you mean "block it".  The mere presence of another wifi network within range of yours has nothing to do with your network's performance.  Your wifi router is only connected to your network interface, and if your network is secure then only your devices are using your wifi bandwidth.  There could be dozens of adjacent wifi networks, as often happens in a tightly spaced residential neighborhood or in an apartment complex - but they each are independent and separate networks.

If you are in an apartment or some building that shares a single switch to the outside world, and you have access to that switch, yes, you can block any given wifi node at the switch so it does not take up any bandwidth on the shared connection to the internet.  Otherwise, you cannot "block" a nearby wifi network as there is nothing to block - it is not connecting to your ISP gateway so not sharing your intranet bandwidth.

Odds are your network slows down for everyone with accounts with the same ISP in that same area at the same time each day.  It would be because of the volume of traffic on the ISP's equipment, not because of the mere presence of adjacent local wifi networks.

From my home, I can "see" at least 8 other wifi networks.  They are all locked down and secure, as is mine, so nobody is on anybody-elses but their own.

Occasionally, you could see signal interferance from a nearby wifi router, but in my experience that is pretty rare (most interferance is usually from other things entirely - microwave ovens, older cordless telephones, that sort of thing).  Even then, channel interferance usually expresses itself as bad connections, dropped packets and so forth, not network speed so much as just bad network connections.

My AT&T fiber connection peaks in speed at around 20-22Mbps in evening or night hours, but on weekdays during business hours, it drops to as low as 12-15Mbps - just volume of traffic effects on my internet connection.

Try installing the dashboard widget following the link below. It will tell you all the wifi hotspots in your vicinity, including you own, and what channels they are using. Then set your router to a channel that avoids as much as possible the other channels being used. I did this and it made a noticeable improvement to connectivity and speed.

http://www.apple.com/downloads/dashboard/networking_security/airlock_danielpimle y.html

Sorry, for some reason the link did not display fully

http://www.apple.com/downloads/dashboard/networking_security/airlock_danielpimle y.html

Is your network listed as a "Preferred Network"?

     ->System Preferences->Network->[Advanced...]

Also, uncheck "Remember networks this computer has joined"

here is the full link

http://www.apple.com/downloads/dashboard/networking_security/airlock_danielpimle y.html

Thanks. For my education, how did you insert the full link - I did a copy and paste but I assume that the space towards the end of the link stopped it fully copying as a link?

use the Insert Link button

Hmm. I am using an iPad and when I select the advanced editor, it flashes up the editing menu and then it disappears.

You may want to check if the adjacent networks are using the same WiFi channel you are using.

Try a utility such as iStumbler.

For 2.4GHz frequences, you want to be on a channel all by yourself and 5 channels away from the strongest signals.  That means channels 1, 6, or 11 typically.  Outside the US, some countries allow channels 12 & 13, so you may play different 5 channel apart games.

The 5GHz frequencies are better spaced so as long as you are not using the same channel, all is OK.

If you are using channels that conflict, then you can get interference that may slow down your WiFi.

Another approach is to make an Ethernet connection instead of WiFi, and see if your performance improves.  If it does, then it may be WiFi interference.  If it does not improve, then it may be how fast your Internet connection is working.

I know a lot of people using Cable based broadband that see performance slow down when school lets out and the neighborhood kids start watching YouTube videos and chewing up the shared neighborhood bandwidth.