Is it possible to disable beast lightmap at runtime?

Once an object is lightmapped, it's flagged as such internally and therefore it'll always use the lightmap's data to get light information at rendering time.

However if you look at compiled surface shaders (Add

#pragma debug

in the pragma declarations of your surface shader to be able to read the generated shader passes in plain readable code) you will see where specific calls to getting lightmap info are made. This mean that you could potentially deconstruct a surface shader (copy/paste the generated bits of code) and reconstruct it with adding a control on how lightmap data is used, through a 0-1 float or something, which will use lightmapping data when the float is at 0 and 'classic' lighting code when at 1.

So, with a great deal of patience and some serious shader knowledge, yeah, it's possible. Otherwise, no, you cannot do this.