You can purchase a reserved slot, which also includes auto balance immunity.
If you are switched while alive, that means that the plugin kept the teams unbalanced for as long as it is set to. It will not leave the teams unbalanced, so if there is no valid swap target after a certain amount of time, it will pick someone who is alive.
The reserved slot removes any non-immune spectators, followed by active players with the highest ping.
sm_reserve_kicktype <0|1|2> - Only available in SourceMod 1.1 or higher.
This controls how a client is selected to be kicked (only relevant to sm_reserve_type 1/2)
Clients with reserve slot access or the override 'sm_reskick_immunity' are always immune to being kicked. Spectating clients are chosen before playing clients.
0 - Highest Ping
1 - Highest Connection Time
2 - Random Player
We use sm_reserve_kicktype 0