The pasuk teaches us in Paeshas Re'ah that one should destroy idols but it is forbidden to do so to holy items ["lo taasun keyn lashem Elokeichem" – 12/3].

The Mishna in Gittin [32a] teaches that a husband may void a get that he wrote for his wife [I call it the sugya of " 'Get' out of here"]. Is it possible for a sofer stam to void a sefer Torah that he wrote, by declaring it unholy [lish'ol al hesheym]?

Interesting question! Tosphos [Gittin 32b] thought of it already and they ruled NO! It doesn't work. Some suggest that if it would be effective [against the opinon of Toshphos] to nullify the holiness of the sefer Torah, then one who does so, would be guilty of erasing G-d's holy name. Rav Yosef Engel compares this to uprooting ones declaration of Terumah. Such a person is guilty of the sin of "meabeid Terumah" – destroying Terumah. The Rogchover agrees that it is a no-no.

The Shulchan Melachim writes that it is possible that if one writes the name of Hashem it is permitted to nullify it within two seconds of the writing [affectionately known in Talmudic literature as "toch kidei dibbur"] but after this time has elapsed "eeeeeeeee" buzzer sounds and it is too late.

The Minchas Chinuch [437/11] derives from the aforementioned Tosphos that only with a verbal declaration it is impossible to nullify the name of G-d that was written lishmah. But if one would do a maaseh [concrete act] then it would be effective. An example of this would be adding a letter to the name or filling in dots [nekodot]. See there.

The Kli Chemdah [page 66] suggests that if we assume that it is POSSIBLE to uproot the holiness of G-d's name, it would be PERMITTED do so. A proof for this is the halacha that one may uproot a declaration of his possessions to the Beis Hamikdash [sheailah bi'hekdesh] and there is no prohibition of destroying kedusha ["lo taasun keyn"]. Similarly, asserts the Kli Chemda, it is permitted to uproot the sanctity of G-d's name in a sefer Torah.

The Shiras Yitzchak adds that the source for the prohibition against erasing G-d's name is the pasuk that says that one should PHYSICALLY destroy idols but one may not do so to Hashem's name. From this we may derive that just as the mitzva is to physically destroy the idols, so too the prohibition is against PHYSICALLY obliterating Hashem's name. So at least on a Biblical level a mere verbal voiding of the sanctity of G-d's name would seem to be permitted.

In conclusion there is a dispute if it is permitted to uproot the holiness of a sefer torah [if we assume that such an act is possible]. According to Rav Yosef Engel and The Rogochover Gaon – it is forbidden. According to the Kli Chemdah it is permitted.