Turning on and off pin change interrupts ATtiny85
First of allsei() and cli() enable/disable all interrupts, not just the pin change ones. Unless you don't want any interrupts at all, you shouldn't be using cli() when trying to disable pin change interrupts.

Clearing the PCIE bit in GIMSK is sufficient to turn off pin change interrupts. You don't even need to clear the PCMSK register.

Obligatory excerpt from datasheet (page 51):


  When the PCIE bit is set (one) and the I-bit in the Status Register (SREG) is set (one), pin change interrupt is enabled. Any change on any enabled PCINT[5:0] pin will cause an interrupt. The corresponding interrupt of Pin
  Change Interrupt Request is executed from the PCI Interrupt Vector. PCINT[5:0] pins are enabled individually by the PCMSK0 Register.


Pin change interrupts are only enabled when the PCIE bit in GIMSK is set (one). Otherwise they are disabled.

The global interrupt flag in the SREG must also be set for interrupts to occur (sei()), however if you are already using interrupts in your design this bit is probably already set.