Add them to your censor or noplay list!
Censor will eliminate all communication from that player and it will block players challenging you or accepting your seeks. This will have no effect if you play in tournaments, 1-minute, 3-minute, 5-minute, 15-minute, 45-minute or 960 pools.
Noplay will only block players challenging you as explained in censor.
Command Line Method for Censor and Noplay:
Open your Main console by clicking on the Main Tab under your Channels Window.
To add a player to your censor list, type: +censor username in the main console. Example: +censor Fred
To remove a player to your censor list, type: -censor username in the main console. Example: -censor Fred
Type: =censor to see the players currently on your censor list.
For noplay you can use a similar system.
For the Dasher helpfile on censor CLICK HERE.
For info on how to report abuse with Dasher CLICK HERE.