I think that the link is created by the system when they are creating a timesheet so your evaluation criteria returns TRUE. Before trying to solve this, my suggestion is to look into using the 8.2 People can assign themselves to work items system setting. Setting it to Don't Allow might be able to solve the issue.
Alternatively, you could try to use the following to see if the link has just been created and treat that as self-assignment via timesheet creation: IsNew(GetLink("ResourceLink", CurrentObject(), CurrentUser()))
If that does not work, you could also compare the ResourceLink creation time with Now() to determine if it was just created.
I hope this helps.