Why is placing one’s hand (on Hashem’s name) not regarded as a chatzitza?

Stepping on the creature while walking is muttar even according to Rebbi Yehuda, since the Rabbanan deferred their issur in a situation of Mazikin.  Why don’t we say the same according to Rebbi Shimon; that the issur miderabanan (of Melacha she’ina tzericha legufa) is muttar in cases of Mazikin, thereby allowing killing them on sight?