I could help set filtering MAC in my GL-MT300n-V2, is possible?
In principle, YES. Check hostapd.conf ; there you can exclude/include certain MACs. Or, to be more flexible, use freeradius+macauth together with hostapd.conf.
Use firmware 3.0 and blocks clients based on IP and Mac easily.
firmware v3 only appears on the download page in the folder testing, is it safe to install this firmware?
You can also try to add special firewall cmds to block certain MACs.
google for “iptables filter MAC”
As the MT300n is not the most powerful device, you might consider building your own image for it, only including packages, required for your purpose.
I installed the firmware 3.008-1123 beta in my gl-mt300n-v2, and as you can see it is very easy to block users through the web interface, but the firmware is still in beta and presents problems, for example every so often I can not access to the web interface, only to LUci, and I have to turn it off and turn it on to access, the red LED stopped working, where the clients disappear the name and only shows asterisks, disappears the option to share samba files in luci, disappears the installed packages that I had, (nodegsplash, the Spanish language, etc), I would like to correct this because I think the update is very good but this way it is not possible to work, so I had to go back to firmware v2-2.271 which is the one stable.