Knockdown of long noncoding RNA (lncRNA) - how is it done?
Knockdown of lncRNA in mammals is not done via RNAi. Instead, one transfers antisense DNA oligos which bind to the RNA. This triggers the action of the RNase H enzyme, which degrades RNA-DNA duplexes. It degrades the lncRNA.

UPDATE: For reference, I learned about this from a seminar, and it is not very well documented, but after some literature search I found this paper to quote:


  Although we (Fig. 3A) and others (42, 49) have used siRNA to knock down NEAT1 lncRNA and although the knockdown of strictly nuclear RNAs has been well described (53, 54), we remained concerned that because NEAT1 is mostly a nuclear RNA (55), it may not be very efficiently targeted by the RNAi machinery. Nuclear RNAs, however, are proficiently targeted using complementary antisense (AS) oligodeoxynucleotides that recruit nuclear RNase H activity to degrade the RNA (56). Hence, we also employed complementary AS oligodeoxynucleotides to knock down NEAT1 lncRNA in HIV-1 NL4-3-infected Jurkat cells (Fig. 4C, left). The AS approach reduced NEAT1 (Fig. 4C, left) and increased HIV-1 p24 production (Fig. 4C, right), providing results consistent with those from siRNA-mediated knockdown of NEAT1 (Fig. 4A and B).