The English "abuse" is adequately translated by the German "Missbrauch". In both languages it may have a sexual connotation, but if you read it in a mathematical text you would never interpret it like that. It often occurs as a compound with other nouns, for example "Alkoholmissbrauch" or "Machtmissbrauch".
An example of a mathematical text containing the phrase is https://www.math.uni-hamburg.de/home/riemenschneider/anvorl3.pdf p.568 (the spelling is old orthography). It seems to me that the following variants are not bad:
Unter Missbrauch der Notation schreiben wir $\mathbb{Q} \subset \mathbb{R}$
Wir schreiben missbräuchlich $\mathbb{Q} \subset \mathbb{R}$
Wir verwenden die missbräuchliche Schreibweise $\mathbb{Q} \subset \mathbb{R}$
Note that the following does not sound nice:
- Unter Missbrauch der Schreibweise schreiben wir $\mathbb{Q} \subset \mathbb{R}$
However, perhaps you should also ask a question in https://german.stackexchange.com/
Edited:
The phrase does not occur that frequently in German. I cannot remember that I have ever seen it in older literature, but I admittedly I am not sure. This indicates that it might be an Anglicism. Another hint is this:
A Google search with "Missbrauch der Schreibweise" produces only a few results, but if you do it with "Missbrauch der Notation" you will get a lot more. And the latter is the most literal translation of "abuse of notation".