Hi huyilin,

赶不开.

赶 means"to drive away; expel; whisk something off."

开 means"to disperse; separate."

So like the flies, it's hard to drive them away.

In this context, people also say "赶不走"

Also you will hear 甩不开 quite often (shuai3 means to leave someone behind, to throw away) cannot leave someone/ something behind.