Few things to start:
DejaVu Sans has long standing constantly apearing and disapearing bugs with my language diacritic symbols. Some time ago it was unbearable, now it reemerges only in few places. Now it is one letter and only appears in DejaVu Sans Bold and Oblique versions, but it's status hadn't change for 1.5 years and is unpleasant to view. Also bug with too low symbol _ is cripling blow too.
How can I disable, blacklist and substitute properly DejaVu Sans fonts, what viable substitute fonts could be used? Diacrite symbol problem was seen only in DejavuSans family so far. What I could try?
I read wiki entry about font aliases, but is there any way to blacklist font entirely, that it couldn't be used by any Desktop Environment?
Why don't you just uninstall it? ttf-dejavu? As long as you have a replacement ttf-font this should satisfy the app requirements that likely pulled it in.
I laugh, yet the joke is on me
You can use a fontconfig rule - here's an example from Infinality:
<match target="font"> <test name="family"><string>Terminus</string></test> <edit mode="assign" name="family"><string>Terminus (ttf)</string></edit> </match>