Oh, sorry I understood it the wrong way!
There are many ways to change that as in
- disabling chat(they could still PM if those aren't turned off while sparring), but may not make them start to spar when facing a friend
- lowering down the time of a spar, but this may "destroy" good spar games which are really fun
- etc.
But all those ways to change that may destroy the actual fun sparring is. The best way would be to add a rule which forbidds you to evade sparring while being in a spar, but this sounds ******ed and pathetic..
I think you just have to deal with it, no matter how much it is annoying.